Integrate keymap-drawer into flake

[MattSturgeon]

Refactor keymap-drawer so that is is part of the flake.

• [x] Build keymap-drawer using poetry2nix
• [x] Re-implement CI logic/scripts as nix builds
• [ ] Port CI to flake builds
• [ ] Remove old scripts
• [ ] Remove symlink hack-fix, needs https://github.com/caksoylar/keymap-drawer/issues/84
• [ ] Fix SVG icons (can't download in a pure env)
  • [ ] glyph_urls might support file: URLs? If not open upstream request.
  • [ ] use_local_cache may need to be disabled, depending on where it writes to