contrib: rav1e: always use the native cargo/Rust compiler for vendoring

There should be no difference:

- the vendor system is meant to pick the source needed to rebuild the package
  for all targets, it doesn't require a target, in fact there is no "--target" option.
- someone even made a tool to do vendoring for a specific target:
  https://users.rust-lang.org/t/is-it-possible-to-vendor-dependencies-only-for-a-given-target/90261/2
- before and after the patch the rav1e-vendor-build folder is the same
- using CARGO_NATIVE on Linux I still get a ton of windows packages in the
  rav1e-vendor folder

It may start a little faster while the cross compiler is being downloaded.