The rust-toolchain file can also store an exact nightly snapshot
(instead of just "nightly"), so we can store whatever snapshot that
rustc_codegen_cranelift is known to work with.
This also lets us add a new feature to `cargo.sh` to let it use the
exact same nightly snapshot as cg_clif. If there's a nightly compiler
mismatch, you get a confusing error message like:
error: couldn't load codegen backend "librustc_codegen_cranelift.so":
"librustc_driver-681e2b4f66c73d3e.so: cannot open
shared object file: No such file or directory"
So doing this automatically in cargo.sh is useful.
