cmd/go: omit VCS stamping during bootstrap

cmd/go: omit VCS stamping during bootstrap

cmd/dist can't easily hard-code -buildvcs=false because not all
versions of cmd/go supported for bootstrapping recognize that flag.

However, we don't want to stamp the bootstrap binaries: the stamping
is redundant with the VERSION file writted during bootstrapping (which
is why it is normally omitted for standard-library packages and
commands), and it may also interfere with building the Go repo from a
source tarball or zip file.

Fixes #54852.

Change-Id: If223f094af137c4c202d6bf622619bd2da397ec4
Reviewed-on: https://go-review.googlesource.com/c/go/+/432435
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>