I rarely need a debug build of cg_clif, and even if I actually need one,
using package overrides in Cargo.toml to only do a debug build for the
rustc_codegen_cranelift crate works much better.
This avoids building cranelift twice in rust's CI and is a lot easier
than trying to make building of codegen backends work from within a
cargo invocation done by rust's build system.
This can be done by installing the nightly specified in
rust-toolchain.toml and then pointing the CARGO, RUSTC and RUSTDOC env
vars to the right executables.
