Fix CI breakage (#2273)

Drop `master` channel, as it is currently broken.

## Pre-launch Checklist

- [x] I read the [Effective Dart: Style] _recently_, and have followed
its advice.
- [x] I signed the [CLA].
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-devrel
channel on [Discord].

<!-- Links -->
[Effective Dart: Style]:
https://dart.dev/guides/language/effective-dart/style
[CLA]: https://cla.developers.google.com/
[Discord]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Chat.md

.github/workflows/flutter-ci.yml

@@ -23,7 +23,8 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        flutter_version: [stable, beta, master]
+        # Add master back once https://github.com/flutter/flutter/issues/160558 is fixed
+        flutter_version: [stable, beta]
         os: [ubuntu-latest, macos-latest, windows-latest]
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683