Largely taken from https://input-output-hk.github.io/haskell.nix/tutorials/getting-started-flakes.html.

Builds from x86 Linux, for x86 Musl Linux or Windows.

Taken from https://input-output-hk.github.io/haskell.nix/tutorials/cross-compilation.html#static-executables-with-musl-libc.

It turns out files under `extra-source-files` are not found at compile time (`file-embed`) unless they're in `rsc` (and not as symlinks), and known to Git (staging makes a difference, which confused me for a while, e.g. in georgefst/hello-hs@fa41589).

This is primarily so that we can remove a dependency on the GPL-licensed libgmp. See https://github.com/input-output-hk/haskell.nix/pull/1784/commits.