cmd/go: fix retrieving Mercurial commit timestamp under Windows

cmd/go: fix retrieving Mercurial commit timestamp under Windows

Use "hgdate" since the strftime filter is unsupported by Mercurial under
Windows.

Fixes #49841

Change-Id: I300898e51e324147aaf1bfe12ed17dea4bdd183d
Reviewed-on: https://go-review.googlesource.com/c/go/+/380077
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Trust: Jeremy Faller <jeremy@golang.org>
Trust: Michael Knyszek <mknyszek@google.com>