Enable RBE across platforms in a simple manner.

The logic of selecting the correct configuration file has been moved
into the build system.

Bug: b/296994239
Change-Id: I1a2567f10f38542f9ec8f74ada4d5494df389a60
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/355382
Reviewed-by: William Hesse <whesse@google.com>
Commit-Queue: Jonas Termansen <sortie@google.com>
3 files changed