check: Fix branch name for clone of Flutter repo in CI

Otherwise the clone doesn't know about the branch name `main`,
which would confuse the logic in a check we're about to add.

Author: gnprice

.github/workflows/ci.yml

@@ -23,7 +23,7 @@ jobs:
       # pubspec is satisfied.  It's uncommon for flutter/flutter to go
       # more than 100 commits between tags.  Fetch 1000 for good measure.
       run: |
-        git clone --depth=1000 https://github.com/flutter/flutter ~/flutter
+        git clone --depth=1000 -b main https://github.com/flutter/flutter ~/flutter
         TZ=UTC git --git-dir ~/flutter/.git log -1 --format='%h | %ci | %s' --date=iso8601-local
         echo ~/flutter/bin >> "$GITHUB_PATH"