make a github action workflow for building the system automatically and pushing it to cachix

[RCMast3r]

https://discourse.nixos.org/t/nix-github-actions-aarch64/11034/5

desired workflow:

• automatic rebuild of top-level can be triggered in two ways:

1. update of HT_CAN or data_acq repo will trigger nix flake update of HT_CAN input here (common)
2. update of hytech_nixos flake itself (medium rare)

• updating of TCU:
  a user should only have to run one command to push the store path to the car. While doing this, the command will get the top level store path from remote and serve it to the car. The user will have to be connected to both the car and to internet.

nix copy --from (remote) --to (tcu)

since the TCU does not have an internet connection