cmd/go: don't let script grep commands match $WORK

If $WORK happens to contain the string that a stdout/stderr/grep
command is searching for, a negative grep command will fail incorrectly.

Fixes #27170
Fixes #27221

Change-Id: I84454d3c42360fe3295c7235d388381525eb85b4
Reviewed-on: https://go-review.googlesource.com/131398


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>