contrib: rav1e: require rustc+cargo to be available before we get the vendor tarball

Since we want to build the vendor tarball if it's not there, we require rustc+cargo to be available when calling rav1e-vendor-build. However we cannot do the dependency in the rav1e-vendor-build target since it would be handled in another process that handles the contrib dependencies.

That means we will download the native cargo+rustc during the fetch stage. Which is actually a good thing, except it won't be in our tarballs.

Co-authored-by: Martin Storsjö martin@martin.st