flakes/manual: add authentik-nix

[luochen1990]

No description provided.

[raboof]

Seems to fail with:

             error: attribute 'pkgs' missing
             at /nix/store/8rv7qar60c9p7hzwfa02syvl4yzrgmsm-source/lib/modules.nix:554:28:
                553|         addErrorContext (context name)
                554|           (args.${name} or config._module.args.${name})
                   |                            ^
                555|       ) (functionArgs f);