Use the same upgrade script on Mac (flutter#1865)

This also fetches master that will populate `FETCH_HEAD`

See flutter#1864

Author: fkorotkov

.cirrus.yml

@@ -60,10 +60,10 @@ task:
     image: mojave-xcode-10.2-flutter
   setup_script:
     - pod repo update
-  switch_channel_script:
+  upgrade_script:
     - flutter channel master
     - flutter upgrade
-    - flutter doctor
+    - git fetch origin master
   activate_script:
     - pub global activate flutter_plugin_tools
   create_simulator_script:
@@ -83,4 +83,4 @@ task:
         SIMCTL_CHILD_MAPS_API_KEY: ENCRYPTED[596a9f6bca436694625ac50851dc5da6b4d34cba8025f7db5bc9465142e8cd44e15f69e3507787753accebfc4910d550]
       build_script:
         - ./script/incremental_build.sh build-examples --ipa
-        - ./script/incremental_build.sh drive-examples
+        - ./script/incremental_build.sh drive-examples