Re-expose openXC7 packages and other artefacts

[fricklerhandwerk]

Repository URL

https://github.com/openXC7/toolchain-nix

Notes

They live in a flake, and there needs to be a bit of hacking to splice that in without using our own flake (since we're moving away from doing all the work in the flake itself). It may benefit from a helper function to evaluate flake.nix, which would build on top of our flake-compat fork.