We were encoding an absolute path into our binaries. This works locally
and breaks horribly when run on the bots. Fix this by rebasing the path
to the root_out_dir.

Bug: angleproject:3630
Change-Id: I4044d1228ee951f2192e83167deb92a0ffe92a52
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2103672Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>