Add python environment to rpi nixos flake

[freddydumont]

The rpi image created in CI should be fully functional. We should be able to flash the pi with it, and it should be ready to accept requests as soon as its plugged into the network.

There's an overlap between regular flakes and devenv that we need to figure out.

• python env
• launch python program on boot (devenv or systemd)
• caddy web server
• other dependencies?

Update

Assuming we're going with #34, we'll likely only need to ensure the repo is in the pi's filesystem with 2 systemd configurations:

• run the python app on boot
• every night, git pull main and restart the server if there were changes