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/nix-darwin/x86_64-darwin/apavel-a01/home/apavel.nix

@@ -23,14 +23,6 @@
     inputs.zls-overlay.packages.x86_64-darwin.default
   ];
 
-  nixpkgs = {
-    overlays = [
-      inputs.zig-overlay.overlays.default
-    ];
-
-    config.allowUnfree = true;
-  };
-
   programs.git = {
     enable = true;
     userName = "apavel";