This is as part of an effort to move environment variables into config
files so that configs can be tied down to source.
Test:
Ran a build with "ANDROID_BUILD_ENVIRONMENT_CONFIG=googler m" which ran
a non-RBE build (since this is vanilla aosp).
Also ran a build with "ANDROID_BUILD_ENVIRONMENT_CONFIG= m" which ran a
non-RBE build.
https://paste.googleplex.com/5637282880028672
Bug: b/194679562
Change-Id: I416e8da75f84aa2b53995f525cf50501488dc972
51c4091309