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/nixos/x86_64-linux/jeeves/configuration.nix

@@ -22,15 +22,6 @@
     # localStorageDir = "${inputs.self}/secrets/rekeyed/${config.networking.hostName}";
   };
 
-  nixpkgs = {
-    hostPlatform = "x86_64-linux";
-    config = {
-      allowUnfree = true;
-    };
-    overlays = [
-    ];
-  };
-
   networking.hostName = "jeeves";
 
   boot = {

machines/nixos/x86_64-linux/jeeves/home/jeeves.nix

@@ -5,12 +5,6 @@
     inputs.wired.homeManagerModules.default
   ];
 
-  nixpkgs = {
-    overlays = builtins.attrValues outputs.overlays;
-
-    config.allowUnfree = true;
-  };
-
   home = {
     username = "jeeves";
     homeDirectory = "/home/jeeves";