Builds carrier-x86_64.flatpak from the linux job's already-built binary
rather than recompiling inside flatpak-builder's sandbox (which can't
see the shared cargo cache anyway, making an in-sandbox build pure
duplicate work). com.randogoth.carrier.release.json is a separate
manifest from the main one, which stays as-is for local
`devbox run package` dev-from-source builds.
