Pin the release path's macOS runners to macos-15 too: release.yaml's transcripts list still named macos-latest, whose ucm artifact no longer exists since the build moved to macos-15 (unisonweb#6309), and bundle-ucm.yaml built on macos-latest, whose macOS 27 SDK GHC's linker can't link against. Release asset names come from the runner's OS and architecture, so they don't change.