Don't use 'flutter upgrade' on Cirrus (#429)

This command isn't intended for CI use, and is also slower due to
downloading artifacts that will be immidately discarded.

See flutter/flutter#86037

.cirrus.yml

@@ -12,9 +12,17 @@ tool_setup_template: &TOOL_SETUP_TEMPLATE
 
 flutter_upgrade_template: &FLUTTER_UPGRADE_TEMPLATE
   upgrade_flutter_script:
-    - flutter channel $CHANNEL
-    - flutter upgrade
-    - flutter doctor
+    # Ensure that the repository has all the branches.
+    - cd $FLUTTER_HOME
+    - git config remote.origin.fetch "+refs/heads/*:refs/remotes/origin/*"
+    - git fetch origin
+    # Switch to the requested branch.
+    - git checkout $CHANNEL
+    # Reset to upstream branch, rather than using pull, since the base image
+    # can sometimes be in a state where it has diverged from upstream (!).
+    - git reset --hard @{u}
+    # Run doctor to allow auditing of what version of Flutter the run is using.
+    - flutter doctor -v
   << : *TOOL_SETUP_TEMPLATE
 
 task: