Restructure flake.nix for a simpler config for building different images off the same NixOS config.

nix/configuration/roles/iso_mount/default.nix

@@ -32,14 +32,10 @@ in
     };
   };
 
-  config = lib.mkIf config.me.iso_mount.enable (
-    lib.mkMerge [
-      {
-        environment.systemPackages = [
-          iso_mount
-          iso_unmount
-        ];
-      }
-    ]
-  );
+  config = lib.mkIf config.me.iso_mount.enable {
+    environment.systemPackages = [
+      iso_mount
+      iso_unmount
+    ];
+  };
 }