Skip build in `nix flake check` on macos runner

Even after no-build mode
evaluating flake...
checking flake output 'formatter'...
checking flake output 'devShells'...
checking derivation devShells.aarch64-darwin.default...
derivation evaluated to /nix/store/5x9ipydgda3vjd[8](https://github.com/kachick/dotfiles/actions/runs/10224406114/job/28292102386?pr=718#step:9:9)iyvhgxkcypm8ghx48-nix-shell.drv
checking flake output 'apps'...
checking flake output 'nixosConfigurations'...
checking NixOS configuration 'nixosConfigurations.moss'...
error:
       … while checking flake output 'nixosConfigurations'
         at /nix/store/lya2qvjid[9](https://github.com/kachick/dotfiles/actions/runs/10224406114/job/28292102386?pr=718#step:9:10)msmyiyrx3v8rmkn0imnibs-source/flake.nix:130:7:
          129|
          130|       nixosConfigurations =
             |       ^
          131|         let

       … while checking the NixOS configuration 'nixosConfigurations.moss'
         at /nix/store/lya2qvjid9msmyiyrx3v8rmkn0imnibs-source/flake.nix:170:[11](https://github.com/kachick/dotfiles/actions/runs/10224406114/job/28292102386?pr=718#step:9:12):
          169|         {
          170|           "moss" = nixpkgs.lib.nixosSystem (
             |           ^
          171|             shared // { modules = shared.modules ++ [ ./nixos/hosts/moss ]; }

       … while evaluating the option `system.build.toplevel':

       … while evaluating definitions from `/nix/store/k251vbbhlk91xwjkxnma7g545i5avhah-source/nixos/modules/system/activation/top-level.nix':

       … while evaluating the option `warnings':

       … while evaluating definitions from `/nix/store/k251vbbhlk91xwjkxnma7g545i5avhah-source/nixos/modules/system/boot/systemd.nix':

       … while evaluating the option `systemd.services.home-manager-kachick.serviceConfig':

       … while evaluating definitions from `/nix/store/k[25](https://github.com/kachick/dotfiles/actions/runs/10224406114/job/28292102386?pr=718#step:9:26)1vbbhlk91xwjkxnma7g545i5avhah-source/flake.nix':

       … while evaluating the option `home-manager.users.kachick.home.file."/home/kachick/.config/systemd/user/gpg-agent.socket".source':

       … while evaluating definitions from `/nix/store/hxhym8c5xz6dxkl3d9yppiwlnzk3khn7-source/modules/misc/xdg.nix':

       … while evaluating the option `home-manager.users.kachick.xdg.configFile."systemd/user/gpg-agent.socket".source':

       … while evaluating definitions from `/nix/store/hxhym8c5xz6dxkl3d9yppiwlnzk3khn7-source/modules/systemd.nix':

       … while evaluating definitions from `/nix/store/hxhym8c5xz6dxkl3d9yppiwlnzk3khn7-source/modules/services/gpg-agent.nix':

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error: invalid regular expression ''
kachick / dotfiles

Skip build in `nix flake check` on macos runner #718
