[automerger skipped] [DO NOT MERGE] Add a default value for the RBE_log_dir...

[automerger skipped] [DO NOT MERGE] Add a default value for the RBE_log_dir variable. am: 8229916e am: 2a3f0525 -s ours am: dcb941f2 -s ours

am skip reason: Change-Id Id93dd80119037b594e5f8c370f4ac2fbc12a5144 with SHA-1 26195be9 is in history

Original change: https://googleplex-android-review.googlesource.com/c/platform/build/soong/+/12595317

Change-Id: I4f38173b8a6e4fa595b66b462cae9eef82810b0b