Builds macos-arm64 and linux-x86_64 release binaries in parallel and
uploads them as build artifacts; a single release job gated on both
then downloads and publishes them to one Forgejo release. Splitting it
this way avoids a real race in actions/forgejo-release itself: its
script always does a blind create-then-upload for the tag with no
merge path, so two parallel callers against the same tag either
hard-fail or stomp each other's uploaded assets.
