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 <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>