Allow overrides for RBE_log_dir

We should override log-dir with the environment variables if its set.
This is needed in our integration tests and generally to allow for
user-overrides.

Test: RBE_log_dir=`pwd`/rbelogs mmma external/boringssl
Change-Id: I3d316a2d60122c42f5c3b5dfede4037362a7b5f1