Hydra not populating with cross compile builds is the bane of my existence.

I'm using `clang` from `pkgs.pkgsCross.musl64.llvmPackages_latest.stdenv` to cross-compile Rust binaries from ARM macos to `x86_64-unknown-linux-musl`. It _works_, but every time I update my `flake.nix` it rebuilds *the entire LLVM toolchain*. On an M2 air, that takes something like 4 hours. It's incredibly frustrating and makes me wary of updating my dependencies or my flake file.

The alternative is to switch to dockerized builds but:

1) That adds a fairly heavyweight requirement to the build process

2) All the headache of writing dockerfiles with careful cache layering

3) Most importantly, feels like admitting defeat.