mullvad: Move script into system module

It doesn’t make sense to install the configuration files system-wide but
the script only for the user.

modules/mullvad/default.nix

@@ -47,9 +47,26 @@ in
   };
 
   config = lib.mkIf cfg.enable {
-    environment.etc = builtins.listToAttrs
+    environment = {
+      etc = builtins.listToAttrs
         (map
           (name: lib.nameValuePair "wireguard/${name}" { source = "${relayConfigFiles}/${name}"; })
           (lib.attrNames relayConfigs));
+
+      systemPackages = lib.singleton (pkgs.stdenv.mkDerivation {
+        name = "mullvad-on-demand";
+
+        src = ./mullvad.sh;
+
+        dontUnpack = true;
+        dontBuild = true;
+
+        installPhase = ''
+          runHook preInstall
+          install -D $src $out/bin/mullvad
+          runHook postInstall
+        '';
+      });
+    };
   };
 }