modules: system: nix: override '<nixpkgs>'

Instead of appending to the default value.

This makes overriding some values that are defined as the default value easier.

modules/system/nix/default.nix

@@ -10,6 +10,8 @@ in
     addToRegistry = my.mkDisableOption "add inputs and self to registry";
 
     addToNixPath = my.mkDisableOption "add inputs and self to nix path";
+
+    overrideNixpkgs = my.mkDisableOption "point nixpkgs to pinned system version";
   };
 
   config = lib.mkIf cfg.enable (lib.mkMerge [
@@ -29,17 +31,21 @@ in
         self.flake = inputs.self;
         # Use pinned nixpkgs when using `nix run pkgs#<whatever>`
         pkgs.flake = inputs.nixpkgs;
+        # ... And with `nix run nixpkgs#<whatever>`
+        nixpkgs.flake = lib.mkIf cfg.overrideNixpkgs inputs.nixpkgs;
         # Add NUR to run some packages that are only present there
         nur.flake = inputs.nur;
       };
     })
 
     (lib.mkIf cfg.addToNixPath {
-      nix.nixPath = options.nix.nixPath.default ++ [
+      nix.nixPath = [
         "self=${inputs.self}"
         "pkgs=${inputs.nixpkgs}"
         "nur=${inputs.nur}"
-      ];
+      ]
+      ++ lib.optional cfg.overrideNixpkgs "nixpkgs=${inputs.nixpkgs}"
+      ++ options.nix.nixPath.default;
     })
   ]);
 }