cmd/go: remove work directory on usage error

cmd/go: remove work directory on usage error

    Backport https://golang.org/cl/163237 from the master library:

        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 https://gcc.gnu.org/PR89406

    Reviewed-on: https://go-review.googlesource.com/c/163198

From-SVN: r269086