contrib: rav1e: simplify the vendoring tarball creation

The optional dependency doesn't work. But in the end we can just create the tarball right after the download failed. So the rav1e-$(RAV1E_VERSION)-vendor.tar.bz2 is guaranteed to create a usable tarball (or the build will fail).

Ultimately it should be the job of the download_vendor macro to create it if the download fails.