Define user context in `flake.nix` and refactor install scripts

[dustinlyons]

Currently, we run bin/apply to ask the user for some info, and run a sed replace function to replace various static tokens throughout the code.

These look like %USER%, %HOST_NAME%, etc. in the templates. We should clean these up so that it's defined once in flake.nix, and that input is provided as part of building the system.