Add nix flake check in CI

[kachick]

Noticed when addressing #304

[kachick]

NixOS/nix#6453 ? 🤔

Run nix flake check


evaluating flake...
copying path '/nix/store/1rp9mgpy83k4w11gi58hqfcqhk3nnahp-source' from 'https://cache.nixos.org'...
error:
       … while checking flake output 'packages'

         at /nix/store/hwsdv71bmaqvzbii5viryxc8slw4vr5v-source/lib.nix:73:15:

           72|             {
           73|               ${key} = (attrs.${key} or { })
             |               ^
           74|               // (appendSystem key system ret);

       … while checking the derivation 'packages.x8[6](https://github.com/kachick/dotfiles/actions/runs/6114077316/job/16594875686?pr=305#step:5:7)_64-linux.homeConfigurations'

         at /nix/store/9higzycn3l6h3ra40ki3ckcc8ixvrx5d-source/flake.nix:4[7](https://github.com/kachick/dotfiles/actions/runs/6114077316/job/16594875686?pr=305#step:5:8):9:

           46|
           47|         packages.homeConfigurations =
             |         ^
           4[8](https://github.com/kachick/dotfiles/actions/runs/6114077316/job/16594875686?pr=305#step:5:9)|           {

       error: flake attribute 'packages.x86_64-linux.homeConfigurations' is not a derivation
Error: Process completed with exit code 1.

[kachick]

nix-community/home-manager#1783 🤷‍♂️

[kachick]

https://discourse.nixos.org/t/custom-flake-outputs-for-checks/18877/8 👀

[kachick]

Closing for now