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

rbesetup.sh

@@ -24,8 +24,11 @@ function _source_env_setup_script() {
 }
 
 # This function needs to run first as the remaining defining functions may be
-# using the envsetup.sh defined functions.
-_source_env_setup_script || return
+# using the envsetup.sh defined functions. Skip this part if this script is already
+# being invoked from envsetup.sh.
+if [[ "$1" != "--skip-envsetup" ]]; then
+  _source_env_setup_script || return
+fi
 
 # This function prefixes the given command with appropriate variables needed
 # for the build to be executed with RBE.