Support selecting a different Rust toolchain
At the moment the values passed through the RUSTC and CARGO environment
variables when running configure are ignored and just
as found on the PATH are used.
This is problematic during package builds where we might have to select a specific Rust toolchain, i.e., make sure to always use the system Rust toolchain vs a rustup toolchain.
Signed-off-by: Tobias Kortkamp tobik@FreeBSD.org