cmd/vet/all: build the vet tool within the golang.org/x/tools repository
cmd/vet/all: build the vet tool within the golang.org/x/tools repository

When running cmd/vet/all on multiple builders, the coordinator places
a copy of golang.org/x/tools at a consistent revision in the builders'
GOPATHs. Keep using the consistent revision in module mode by
executing the build from a working directory within that repository.

When not running on a builder, use 'go vet' directly instead of
building an arbitrarily stale vet tool from the user's GOPATH.

Updates #30228

Change-Id: I19bc809247378da98f3e6ac8572f61bda4518143
Reviewed-on: https://go-review.googlesource.com/c/go/+/165740
Run-TryBot: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

akutz/go,akutz/go,CAFxX/go,CAFxX/go,golang/go,cloudflare/go,akutz/go,golang/go,cloudflare/go,josharian/go,cloudflare/go,nawawi/go,CAFxX/go,josharian/go,golang/go,akutz/go,akutz/go,josharian/go,cloudflare/go,cloudflare/go,CAFxX/go,josharian/go,nawawi/go,CAFxX/go,akutz/go,nawawi/go,josharian/go,CAFxX/go,josharian/go,akutz/go,golang/go,golang/go,nawawi/go,golang/go,cloudflare/go,nawawi/go,akutz/go,josharian/go,nawawi/go,cloudflare/go,cloudflare/go,golang/go,CAFxX/go,CAFxX/go,nawawi/go,golang/go,josharian/go,nawawi/go