CI will break whenever subosito/flutter-action is updated

[andrewkolos]

See #812, which was later re-introduced (#816 (comment)).

We could/should do one of the following

1. update the flutter org's allowlist to permit any version of subosito/flutter-action using a wildcard (if we trust it enough--I've not investigated this)
2. stop dependabot from bumping this version (con: we never get any fixes/patches made to it)
3. just handle checking out flutter ourselves, avoiding dependency on subosito/flutter-action altogether (experimented with here: https://github.com/flutter/genui/pull/802/changes/77b9d351abb712ea5e7fcee21a4892562af86db3..a41db5aa98b9edaee21f83e2caf14b886854fdb1)