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
diff --git a/envsetup.sh b/envsetup.sh
index 4f9440e..77b2247 100644
--- a/envsetup.sh
+++ b/envsetup.sh
@@ -1871,6 +1871,16 @@
     fi
 }
 
+# Source necessary setup scripts needed to run the build with Remote Execution.
+function source_rbe() {
+    local T=$(gettop)
+
+    if [[ "x$USE_RBE" != "x" && "$USE_RBE" != "false" ]]; then
+        . $T/build/make/rbesetup.sh --skip-envsetup
+    fi
+}
+
 validate_current_shell
 source_vendorsetup
+source_rbe
 addcompletions