Switch default RBE strategy to remote_local_fallback.

This should help try jobs failing with ResourceExhausted.

Note the snapshot training actions still explicitly pass the racing strategy as an argument that takes precendence over this environment variable.

Change-Id: I9b6ce40ad74fd85ef520c237c74a1c291356b473
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/365940
Reviewed-by: Alexander Thomas <athom@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Alexander Thomas <athom@google.com>
diff --git a/recipe_modules/build/api.py b/recipe_modules/build/api.py
index 6a2c07e..482e849 100644
--- a/recipe_modules/build/api.py
+++ b/recipe_modules/build/api.py
@@ -37,7 +37,7 @@
   def _build_env(self):
     return {} if not self._use_rbe else {
         'RBE': '1',
-        'RBE_exec_strategy': 'remote',
+        'RBE_exec_strategy': 'remote_local_fallback',
         'RBE_proxy_log_dir': self.m.path['cleanup'],
         'RBE_use_gce_credentials': 'true',
         'RBE_use_application_default_credentials': 'false',
diff --git a/recipe_modules/build/tests/test_build.expected/rbe.json b/recipe_modules/build/tests/test_build.expected/rbe.json
index 1dffd54..0965100 100644
--- a/recipe_modules/build/tests/test_build.expected/rbe.json
+++ b/recipe_modules/build/tests/test_build.expected/rbe.json
@@ -9,7 +9,7 @@
     "cwd": "None",
     "env": {
       "RBE": "1",
-      "RBE_exec_strategy": "remote",
+      "RBE_exec_strategy": "remote_local_fallback",
       "RBE_proxy_log_dir": "[CLEANUP]",
       "RBE_use_application_default_credentials": "false",
       "RBE_use_gce_credentials": "true"