builder: Add helper to get the path to a sysroot wrapper. Nits from https://chromium-review.googlesource.com/#/c/268399/. BUG=None TEST=unittests. Change-Id: I9b6ade292ac215fc83bb827ba939b1a717c57c48 Reviewed-on: https://chromium-review.googlesource.com/268679 Reviewed-by: Gilad Arnold <garnold@chromium.org> Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org> Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>