Nix module updates

[brynblack]

Description

This PR updates the NixOS and home manager modules to fix related bugs and issues that were present when using it.

Namely, the user unit service would not run on login, and this PR fixes that. It also reverts back to using manually specified file paths for the password file and other important credentials.

Tasks

• [x] 1. Make service properly start on login
• [x] 2. Remove default directories

Final checklist

• [x] Domain specific tests
• [x] Full tests
• [x] Updated inline-comment documentation
• [x] Lint fixed
• [x] Squash and rebased
• [x] Sanity check the final build

[CMCDragonkai]

When @tegefaulkes or someone similar does the release you should be updating the public pin of PK CLI too @brynblack.

[CMCDragonkai]

Don't forget to announce it on discord when it happens. Or automate it with a discord bot? When can we get a CF-worker controlled bot with arbitrary rules?