cmd/go: fix tests broken in CL 358539

cmd/go: fix tests broken in CL 358539

CL 358539 revised the build-stamp format, and updated the git and hg
tests to match. However, the fossil and bzr tests were missed, and
were not caught on the builders due to the fact that none of the
builder images have the necessary VCS tools installed.

Updates #48802
Updates #49168

Change-Id: I6b9fd0e19b81cb539864c94ab0860f74e7be6748
Reviewed-on: https://go-review.googlesource.com/c/go/+/369743
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>