Since 205d80c75a (findrust: Simplify (#11328), 2025-03-30), we need
to set Rust_COMPILER and Rust_CARGO instead of Rust_TOOLCHAIN (which
is no longer used). Adjust macOS builds accordingly.
When I set only one of the two, the error messages were pretty
unintelligible. But I guess few normal users need to override the
Rust version, and they can always look here.
While at it, enable localization. AFAIK, the only reason why we didn't
do this on macOS were problems related to the gettext shared library /
dependency. We no longer need that, and it's already tested in CI.
