Add the ability to source RBE related scripts from envsetup.sh

This is so that developers don't have to remember to source a separate
script to source RBE related variables.
I'll modify the docs once this change is submitted.

Test:
USE_RBE=true source build/envsetup.sh sources RBE variables.
USE_RBE=false source build/envsetup.sh does NOT source RBE variables.
source build/envsetup.sh does NOT source RBE variables.

Bug: b/193407413
Change-Id: Ib8abf4c307b02fbb0c63b8a62d7aeda8be42d0fb