chore: globally replace ftl in go.mod (#1227)

Whenever we do `ftl init go ...` in a test, by default the new module
will be configured to pull the FTL source from GitHub. This is both slow
and incorrect. This PR changes that to always use the local FTL.

This has confused me many times.

README.md

@@ -18,9 +18,7 @@ brew tap TBD54566975/ftl && brew install ftl
 set -Eeuxo pipefail
 
 just build ftl
-export FTL_ROOT="$(git rev-parse --show-toplevel)"
-export PATH="$FTL_ROOT/build/release:$PATH"
-export FTL_INIT_GO_REPLACE="github.com/TBD54566975/ftl=$FTL_ROOT"
+export PATH="$(git rev-parse --show-toplevel)/build/release:$PATH"
 
 pwd
 

bin/hermit.hcl

@@ -3,5 +3,7 @@ env = {
   "FTL_SOURCE": "${HERMIT_ENV}",
   "OTEL_METRIC_EXPORT_INTERVAL": "5000",
   "PATH": "${HERMIT_ENV}/scripts:${HERMIT_ENV}/frontend/node_modules/.bin:${PATH}",
+  "FTL_INIT_GO_REPLACE": "github.com/TBD54566975/ftl=${HERMIT_ENV}",
+
 }
 sources = ["env:///bin/packages", "https://github.com/cashapp/hermit-packages.git"]