[automerger skipped] [DO NOT MERGE] Set RBE_DIR default value to point to...

[automerger skipped] [DO NOT MERGE] Set RBE_DIR default value to point to prebuilts in tree. am: 83c95dea am: 32b75849 -s ours am: fb318d78 -s ours am: ec67a994 -s ours am: c2722005 -s ours am: dd42daaf -s ours

am skip reason: subject contains skip directive

Original change: https://googleplex-android-review.googlesource.com/c/platform/build/+/12593960

Change-Id: I62e5e3ebef054aa2b42c9740c856be10ba9c5396