cmd/go: add a "don't care about success" operator to script_test

Use that operator to make test_race_install_cgo agnostic to whether GOROOT/pkg is writable.

Updates #37573
Updates #30316

Change-Id: I018c63b3c369209345069f917bbb3a52179e2b58
Reviewed-on: https://go-review.googlesource.com/c/go/+/223746
Reviewed-by: Jay Conrod <[email protected]>

Author: Bryan C. Mills