Fix data race

[ChrisSchinnerl]

Ran into the following race today. Seems to be super rare and related to relaying a shallow copy of a transaction set.

Error: /opt/hostedtoolcache/go/1.23.3/x64/src/runtime/slice.go:355 +0x0
    go.sia.tech/core/consensus.updateProof()
  Error: /home/runner/go/pkg/mod/go.sia.tech/[email protected]/consensus/merkle.go:391 +0x373
    go.sia.tech/core/consensus.(*elementApplyUpdate).updateElementProof()
  Error: /home/runner/go/pkg/mod/go.sia.tech/[email protected]/consensus/merkle.go:414 +0xa4
    go.sia.tech/core/consensus.ApplyUpdate.UpdateElementProof()
  Error: /home/runner/go/pkg/mod/go.sia.tech/[email protected]/consensus/application.go:668 +0x8a
    go.sia.tech/core/consensus.ApplyUpdate.UpdateElementProof-fm()
        <autogenerated>:1 +0x2a
    go.sia.tech/coreutils/chain.updateTxnProofs.func1()
  Error: /home/runner/go/pkg/mod/go.sia.tech/[email protected]/chain/manager.go:646 +0x201
    go.sia.tech/coreutils/chain.updateTxnProofs()
  Error: /home/runner/go/pkg/mod/go.sia.tech/[email protected]/chain/manager.go:649 +0x10e
    go.sia.tech/coreutils/chain.(*Manager).applyPoolUpdate()
  Error: /home/runner/go/pkg/mod/go.sia.tech/[email protected]/chain/manager.go:774 +0x394
    go.sia.tech/coreutils/chain.(*Manager).applyTip()
  Error: /home/runner/go/pkg/mod/go.sia.tech/[email protected]/chain/manager.go:315 +0xfc4
    go.sia.tech/coreutils/chain.(*Manager).reorgTo()
  Error: /home/runner/go/pkg/mod/go.sia.tech/[email protected]/chain/manager.go:382 +0x387
    go.sia.tech/coreutils/chain.(*Manager).AddBlocks()
  Error: /home/runner/go/pkg/mod/go.sia.tech/[email protected]/chain/manager.go:240 +0x1477
    go.sia.tech/renterd/bus.(*Bus).consensusAcceptBlock()
  Error: /home/runner/work/renterd/renterd/bus/routes.go:179 +0x241
    go.sia.tech/renterd/bus.(*Bus).consensusAcceptBlock-fm()
        <autogenerated>:1 +0xb3
    go.sia.tech/jape.Mux.adaptor.func2()
  Error: /home/runner/go/pkg/mod/go.sia.tech/[email protected]/server.go:170 +0x111
    github.com/julienschmidt/httprouter.(*Router).ServeHTTP()
  Error: /home/runner/go/pkg/mod/github.com/julienschmidt/[email protected]/router.go:387 +0xf0b
    go.sia.tech/renterd/internal/test/e2e.newTestCluster.newTestCluster.BasicAuth.func7.func8()
  Error: /home/runner/go/pkg/mod/go.sia.tech/[email protected]/server.go:229 +0xa4
    net/http.HandlerFunc.ServeHTTP()
  Error: /opt/hostedtoolcache/go/1.23.3/x64/src/net/http/server.go:2220 +0x47
    go.sia.tech/renterd/internal/utils.TreeMux.ServeHTTP()
  Error: /home/runner/work/renterd/renterd/internal/utils/web.go:38 +0x36a
    go.sia.tech/renterd/internal/utils.TreeMux.ServeHTTP()
  Error: /home/runner/work/renterd/renterd/internal/utils/web.go:33 +0x32a
    go.sia.tech/renterd/internal/utils.(*TreeMux).ServeHTTP()
        <autogenerated>:1 +0x74
    net/http.serverHandler.ServeHTTP()
  Error: /opt/hostedtoolcache/go/1.23.3/x64/src/net/http/server.go:3210 +0x2a1
    net/http.(*conn).serve()
  Error: /opt/hostedtoolcache/go/1.23.3/x64/src/net/http/server.go:2092 +0x12a4
    net/http.(*Server).Serve.gowrap3()
  Error: /opt/hostedtoolcache/go/1.23.3/x64/src/net/http/server.go:3360 +0x4f
  
  Previous read at 0x00c001802b40 by goroutine 9586:
    go.sia.tech/core/types.EncodeSlice[go.shape.[32]uint8]()
  Error: /home/runner/go/pkg/mod/go.sia.tech/[email protected]/types/encoding.go:136 +0x1ba
    go.sia.tech/core/types.StateElement.EncodeTo()
  Error: /home/runner/go/pkg/mod/go.sia.tech/[email protected]/types/encoding.go:627 +0x193
    go.sia.tech/core/types.SiacoinElement.EncodeTo()
  Error: /home/runner/go/pkg/mod/go.sia.tech/[email protected]/types/encoding.go:645 +0x5b
    go.sia.tech/core/types.V2SiacoinInput.EncodeTo()
  Error: /home/runner/go/pkg/mod/go.sia.tech/[email protected]/types/encoding.go:632 +0x67
    go.sia.tech/core/types.EncodeSlice[go.shape.struct { Parent go.sia.tech/core/types.SiacoinElement "json:\"parent\""; SatisfiedPolicy go.sia.tech/core/types.SatisfiedPolicy "json:\"satisfiedPolicy\"" }]()
  Error: /home/runner/go/pkg/mod/go.sia.tech/[email protected]/types/encoding.go:136 +0x2c2
    go.sia.tech/core/types.V2Transaction.EncodeTo()
  Error: /home/runner/go/pkg/mod/go.sia.tech/[email protected]/types/encoding.go:775 +0x3a4
    go.sia.tech/core/types.EncodeSlice[go.shape.fe8b6bf03dfec8f56d617a3252d3e4338ee7ffced9b7ee384b02376217abfb7c]()
  Error: /home/runner/go/pkg/mod/go.sia.tech/[email protected]/types/encoding.go:136 +0x285
    go.sia.tech/core/gateway.(*RPCRelayV2TransactionSet).encodeRequest()
  Error: /home/runner/go/pkg/mod/go.sia.tech/[email protected]/gateway/encoding.go:364 +0xb2
    go.sia.tech/core/gateway.Object.encodeRequest-fm()
        <autogenerated>:1 +0x4c
    go.sia.tech/core/gateway.withV2Encoder()
  Error: /home/runner/go/pkg/mod/go.sia.tech/[email protected]/gateway/encoding.go:34 +0xcb
    go.sia.tech/core/gateway.(*Stream).withEncoder()
  Error: /home/runner/go/pkg/mod/go.sia.tech/[email protected]/gateway/transport.go:124 +0xa4
    go.sia.tech/core/gateway.(*Stream).WriteRequest()
  Error: /home/runner/go/pkg/mod/go.sia.tech/[email protected]/gateway/transport.go:155 +0x359
    go.sia.tech/coreutils/syncer.(*Peer).callRPC()
  Error: /home/runner/go/pkg/mod/go.sia.tech/[email protected]/syncer/peer.go:90 +0x360
    go.sia.tech/coreutils/syncer.(*Peer).RelayV2TransactionSet()
  Error: /home/runner/go/pkg/mod/go.sia.tech/[email protected]/syncer/peer.go:196 +0x169
    go.sia.tech/coreutils/syncer.(*Syncer).relayV2TransactionSet.gowrap2()
  Error: /home/runner/go/pkg/mod/go.sia.tech/[email protected]/syncer/syncer.go:355 +0x2e