[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 skip reason: subject contains skip directive

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

Change-Id: I3437bd861db3653bb24158bfcd3d0873f9278b99