ci: Work around broken ref for Flutter upstream main

Fixes zulip#1177.

Workaround for:
  flutter/flutter#160558

.github/workflows/ci.yml

@@ -27,6 +27,11 @@ jobs:
         TZ=UTC git --git-dir ~/flutter/.git log -1 --format='%h | %ci | %s' --date=iso8601-local
         echo ~/flutter/bin >> "$GITHUB_PATH"
 
+        # The Flutter tool assumes the tip of tree is "upstream/master":
+        #   https://github.com/flutter/flutter/issues/160558
+        # TODO(upstream): make workaround unneeded
+        git --git-dir ~/flutter/.git update-ref refs/remotes/upstream/master origin/main
+
     - name: Download Flutter SDK artifacts (flutter precache)
       run: flutter precache --universal