cmd/go: assert on more of the version string in TestScript/gotoolchain

cmd/go: assert on more of the version string in TestScript/gotoolchain

The previous assert triggers whenever the 40-character git commit
contains the substring "999", which happens with a probability
decidedly greater than zero.

For #57001.

Change-Id: If0f1bc1a3dd0e6b7e66768d0cf3a79545ee4e5ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/486399
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>