cmd/go: prevent git from fetching during local only mode
Since we added a local context to git lookups, we need to be more
careful about fetching from remote.
We should not fetch when we are stamping a binary because that could
slow down builds.
For #50603
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-windows-amd64-longtest
Change-Id: I81a719b7609e8d30b32ffb3c12a05074c5fd0c22
Reviewed-on: https://go-review.googlesource.com/c/go/+/611916
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Michael Matloob <matloob@golang.org>