"send on closed channel" in buildengine tests

[alecthomas]

This is the first time I've seen this, so either the test or buildengine itself is slightly flaky. Unfortunately the traceback was really tiny, so I'm not sure it captured anything useful.

panic: send on closed channel

goroutine 5776 [running]:
github.com/alecthomas/types/pubsub.(*Topic[...]).Publish(...)
	/Users/alec/go/pkg/mod/github.com/alecthomas/[email protected]/pubsub/pubsub.go:78
github.com/TBD54566975/ftl/buildengine.Watch.func1()
	/Users/alec/dev/ftl/buildengine/watch.go:92 +0x9e8
created by github.com/TBD54566975/ftl/buildengine.Watch in goroutine 5774
	/Users/alec/dev/ftl/buildengine/watch.go:39 +0x104