refactor(flake)!: deduplicate code in configurations flake module

Pass around `perSystem`'s `pkgs` instances to the system configurations
Some more refactoring in `configurations`' `mkHost` functions
Also start using `home-manager`'s `useGlobalPkgs` option
Also use `mkShellNoCC` for default `devShell`
Also `nix flake update`

machines/home-manager/x86_64-linux/__template__/home.nix

@@ -14,32 +14,6 @@
     ./nvim.nix
   ];
 
-  nixpkgs = {
-    # You can add overlays here
-    overlays = [
-      # If you want to use overlays your own flake exports (from overlays dir):
-      # outputs.overlays.modifications
-      # outputs.overlays.additions
-
-      # Or overlays exported from other flakes:
-      # neovim-nightly-overlay.overlays.default
-
-      # Or define it inline, for example:
-      # (final: prev: {
-      #   hi = final.hello.overrideAttrs (oldAttrs: {
-      #     patches = [ ./change-hello-to-hi.patch ];
-      #   });
-      # })
-    ];
-    # Configure your nixpkgs instance
-    config = {
-      # Disable if you don't want unfree packages
-      allowUnfree = true;
-      # Workaround for https://github.com/nix-community/home-manager/issues/2942
-      allowUnfreePredicate = (_: true);
-    };
-  };
-
   # TODO: Set your username
   home = {
     username = "your-username";