CI: Use `case()` function

CI: Use `case()` function

GitHub Actions introduced a new `case()` function in January that works just
like HAProxy’s `iif()` converter (just with an arbitrary number of
expressions). It is more robust than chaining strings with `&&` and `||`,
because it includes proper type-checking.

see https://github.blog/changelog/2026-01-29-github-actions-smarter-editing-clearer-debugging-and-a-new-case-function/#write-more-expressive-expressions-with-a-case-function