Enable racing for ninja+reclient developer builds

Bug: b/288564971
Change-Id: Ibae8f86b7c79b9fa218e0f04c75fe99414c4f48b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4844059
Reviewed-by: Junji Watanabe <jwata@google.com>
Commit-Queue: Ben Segall <bentekkie@google.com>

ninja_reclient.py

@@ -7,6 +7,7 @@ handles the client lifecycle safely. It will automatically start
 reproxy before running ninja and stop reproxy when ninja stops
 for any reason eg. build completes, keyboard interupt etc."""
 
+import os
 import sys
 
 import ninja
@@ -14,6 +15,7 @@ import reclient_helper
 
 
 def main(argv):
+    os.environ.setdefault("RBE_exec_strategy", "racing")
     with reclient_helper.build_context(argv, 'ninja_reclient') as ret_code:
         if ret_code:
             return ret_code