[release-branch.go1.13] cmd/go: add -d flag to mod_get_test
    
    'go get all' was run in this test without -d. This caused some std
    packages to be reinstalled if the test is run in a slightly different
    configuration than make.bash was run. run.bash would fail in some
    situations because of this. Nothing in the cmd/go tests should modify
    installed std or cmd packages.
    
    Updates #36846
    Updates #35459
    
    Change-Id: Idd259a27d55502923b7fc54f361a77f0ac11eea2
    Reviewed-on: https://go-review.googlesource.com/c/go/+/215721
    Run-TryBot: Jay Conrod <jayconrod@google.com>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: Michael Matloob <matloob@golang.org>
    (cherry picked from commit 71bbffbc48d03b447c73da1f54ac57350fc9b36a)
    Reviewed-on: https://go-review.googlesource.com/c/go/+/218597
    Run-TryBot: Carlos Amedee <carlos@golang.org>
    Reviewed-by: Jay Conrod <jayconrod@google.com>