fix: fail gha-done when any required job failed
Previously, the `gha-done` gate job used an `if:` expression that
evaluated to false whenever any needed job reported a failure, which
caused the job to be *skipped* rather than *failed*. GitHub branch
protection treats skipped required checks as non-blocking, so a PR
could be marked mergeable even though one of its test jobs had failed.
Keep the job always running and move the failure check into a step
that explicitly exits 1 when any dependency failed or was cancelled,
so the "GitHub Actions Completed" required check actually blocks the
merge in that case.
Notes: none
Co-authored-by: trop[bot] <37223003+trop[bot]@users.noreply.github.com>
Co-authored-by: Samuel Attard <samuel.r.attard@gmail.com>
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