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:
Ran forrest build with fix for previous build failure - https://android-build.googleplex.com/builds/abtd/run/L72300000950316038

Bug: b/193407413
Change-Id: I16f057086e2ee4f5918816ea70dbb33d0f5e99ab