ci: The `save-state` command is deprecated

[langston-barrett]

CI builds have begun emitting this warning:

The `save-state` command is deprecated and will be disabled soon. Please upgrade to using Environment Files. For more information see: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

This line (and others?) need to be migrated:

what4/.github/workflows/test.yml

Line 53 in 4e37eb4

echo ::set-output name=matrix::$(nix run nixpkgs#swiProlog -- .github/workflows/gen_matrix.pl)