[build][cleanup] Use relative paths to outputs

Eliminating absolute paths to outputs unlocks potential to remote
build with caching.

Bug: 92670
Change-Id: I5bb847417e401b9ec812d60e07eb95376f813c09
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/650754
Reviewed-by: John Bauman <jbauman@google.com>
2 files changed