Review using Cranelift in CI

[kayabaNerve]

Now that rust-lang/rust#81746 was merged, we can use Cranelift in nightly CIs. This would decrease their compile time EXCEPT I believe we only use nightly for fmt/clippy, which wouldn't be affected by this at all.

We could move our tests to nightly, but I'm unsure we want to just for a faster CI.

Any impact on target directory size may be a worthwhile note.

[kayabaNerve]

https://blog.rust-lang.org/inside-rust/2020/11/15/Using-rustc_codegen_cranelift.html

the hope is that cg_clif will eventually be the default backend in debug mode.

[kayabaNerve]

This is cool, and reduced CI times would be great, but whenever we have non-trivial CI times, it's due to the code itself being non-trivial (and therefore we want optimized codegen) or the code is so critical, we have to test with the codegen backend we expect to use. At best, this could be used for say, 3/4 CI runs to reduce the frequency at which a more intensive CI is run. It isn't worth the complexity to Serai to even keep tracking however.