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.