CI: depends on `set-output`

[lishaduck]

GitHub will probably never be able to remove this, but they'd like to. I noticed that CI depends on invertase/github-action-dart-analyzer@v1, when @v3 is the latest version. The old version depends on "soon-to-be-disabled" (for years) functionality, so it'd probably be wise to update.

[spydon]

Do you want to provide a PR to update it?