If ANGLE is inside some other repo but hasn't been built yet, the shader
generator will continue to look for out dirs inside parent repos. Should
only look within the ANGLE repo.

Bug: angleproject:3227
Change-Id: I084414f99eb3db71fe2c7ee4e59206b59f077a5a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1534462Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>