[apple] Fix `gn gen` when $root_build_dir is not below src/

When creating the symlinks to Xcode in $root_build_dir the script
build/config/apple/sdk_info.py was trying to compute a path that
`gn` would understand, however it incorrectly assumed the build dir
was a subdirectory of src/.

Instead pass the value of $root_build_dir to the script and use it
to generate the path which works both when the build dir is below
src/ and when it is located somewhere else.

Fixed: 1340885
Change-Id: Idd932b22669cbba8518f2f5c8269519cca126234
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3736906
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Louis Romero <lpromero@chromium.org>
Quick-Run: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Louis Romero <lpromero@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1019599}
NOKEYCHECK=True
GitOrigin-RevId: 17432a2e2780bc04b263b211723962fcaf4b043e
3 files changed