Use a separate environment variable for RBE_expensive_exec_strategy.

The bots set RBE_exec_strategy=remote_local_fallback, which we don't want to override racing for the Dart actions.

Change-Id: I8739dc04af183fe2011e2589bd5480fd5a2daca1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/511001
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

tools/gn.py

@@ -449,7 +449,7 @@ def AddCommonGnOptionArgs(parser):
                             os.environ.get('DART_RBE') == '1' or \
                             os.environ.get('RBE_cfg') != None)
     parser.add_argument('--rbe-expensive-exec-strategy',
-                        default=os.environ.get('RBE_exec_strategy'),
+                        default=os.environ.get('RBE_expensive_exec_strategy'),
                         help='Strategy for expensive RBE compilations',
                         type=str)