Speed up RBE jobs with "--sandbox_base=/dev/shm".
Speed up RBE jobs with "--sandbox_base=/dev/shm".

Change-Id: I75665e0a75de350e564a6ece341e9ce54de7292c
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/540923
Commit-Queue: Joe Gregorio <b9cf2471ff6d33504cafe8e6f356b732b2cadced@google.com>
Reviewed-by: Kevin Lubick <7cdab2cfab351f23814786ba39716e90eed69047@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot