cmd/go: remove work directory on usage error

cmd/go: remove work directory on usage error

Ensure that cmd/go consistently calls base.Exit rather than os.Exit,
so that we don't incorrectly leave the work directory around on exit.

Test this by modifying the testsuite to run all the tests with TMPDIR
set to a temporary directory, and then check that no files are left
behind in that temporary directory. Adjust a couple of tests to make
this approach work.

Updates #30500
Updates https://gcc.gnu.org/PR89406

Change-Id: Ib6a5fc8a288a6cf4713022baa2b8dfefad62ba34
Reviewed-on: https://go-review.googlesource.com/c/163237
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>