[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 skip reason: Change-Id I3392a0f7ac6de818d0c5ba2e113fcb035c6fa0ae with SHA-1 8f8d1126 is in history

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

Change-Id: Ia9f88e1ccf8092da3cd0184a759c40e68b68fa5e