cargo_util.bbclass: fix for native users
cargo_util.bbclass: fix for native users

Building native packages with cargo means with need a native rust
compiler

cardoe/meta-rust,cardoe/meta-rust,jmesmon/meta-rust,cardoe/meta-rust,cardoe/meta-rust,jmesmon/meta-rust,meta-rust/meta-rust,starlab-io/meta-rust,meta-rust/meta-rust,starlab-io/meta-rust,jmesmon/meta-rust,meta-rust/meta-rust,jmesmon/meta-rust,meta-rust/meta-rust,meta-rust/meta-rust