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.
This allows executing it like cargo clif build if you add it to your
PATH. It also fixes infinite recursion on Windows when invoking it as
Windows includes the current directory in PATH by default.
Fixes#971
