Nix cross-compilation

[samhh]

nix run works on macOS, however because it's built against our host system the binary fails inside of a container:

$ nix run
2023-10-30T14:53:25.596675Z  WARN No .env found
[...]

$ nix build ".#dockerImage" && ./result | podman load
$ podman run mercury
{"msg":"exec container process `/nix/store/zi27abixfpaq0x4czinw34dyjwvb7lrb-mercury-0.0.0/bin/mercury`: Exec format error","level":"error","time":"2023-10-30T14:55:01.480708Z"}

We could potentially use crossSystem ^1, however that loses access to the Nix binary cache, meaning we end up building the universe.

Thus our options for building the container on macOS are:

• Building the universe.
• Running an entire Linux container locally, and then running Nix and the container within that (container-ception).
• Building the binary outside of Nix with cross, and then somehow incorporating that into Nix, or building the image outside of Nix entirely.
• Using an obscure overlay ^2 with its own binary cache.

[samhh]

This may also affect the container itself as per podman image inspect's Architecture.