[infra] Ensure Mac xSAN backfill tasks can find Xcode.

This CL prepares for installing Xcode in a Swarming cache dir from a
CIPD package. https://skia-review.googlesource.com/c/skia/+/144608

After that, I hope to isolate the xSAN libs in the Build task so that
we don't need Xcode on the xSAN test bots. Baby steps.

Change-Id: I61dbe9474eceff42cbfd824c429db382b68a8c35
Reviewed-on: https://skia-review.googlesource.com/145143
Auto-Submit: Ben Wagner <benjaminwagner@google.com>
Reviewed-by: Eric Boren <borenet@google.com>
Commit-Queue: Ben Wagner <benjaminwagner@google.com>
2 files changed