cmd/go: allow '-buildvcs=auto' and treat it as 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.