…s the default

When we added VCS stamping in the Go 1.18 release, we defaulted to
-buildvcs=true, on the theory that most folks will actually want VCS
information stamped.

We also made -buildvcs=true error out if a VCS directory is found and
no VCS tool is available, on the theory that a user who builds with
'-buildvcs=true' will be very surprised if the VCS metadata is
silently missing.

However, that causes a problem for CI environments that don't have the
appropriate VCS tool installed. (And we know that's a common situation
because we're in that situation ourselves — see #46693!)

The new '-buildvcs=auto' setting provides a middle ground: it stamps
VCS information by default when the tool is present (and reports
explicit errors if the tool errors out), but omits the metadata
when the tool isn't present at all.

Updates #51748.
Updates #51999.
Fixes #51798.

Change-Id: Iebc955c2af0abca9b7517f62ca48b1d944eb2df4
Reviewed-on: https://go-review.googlesource.com/c/go/+/398855
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
(cherry picked from commit 4569fe6)
Reviewed-on: https://go-review.googlesource.com/c/go/+/400454
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>

Dockerfile fails with golang-1.18. It needs git until [1] is fixed.
There are two options to fix it:

1. use 1.17 until -buildvcs=auto is added
2. add -buildvcs=false

This PR uses the first option.

[1]: golang/go#51798