fix(sddm): Move pkg derv to it's own file

Nix was complaining that it wasn't a valid package so moved to it's own
file and called with callPackage()

modules/nixos/sddm-rose-pine-pkg.nix

@@ -0,0 +1,21 @@
+{
+  stdenvNoCC,
+  libsForQt5,
+  sources,
+}:
+stdenvNoCC.mkDerivation {
+  pname = "sddm-rose-pine-theme";
+  version = "1.2";
+  dontBuild = true;
+
+  propagatedUserEnvPkgs = [
+    libsForQt5.qt5.qtgraphicaleffects
+  ];
+
+  src = sources.sddm-rose-pine;
+
+  installPhase = ''
+    mkdir -p $out/share/sddm/themes
+    cp -aR $src $out/share/sddm/themes/rose-pine
+  '';
+}

modules/nixos/sddm.nix

@@ -1,6 +1,7 @@
 {
   lib,
   config,
+  pkgs,
   ...
 }: let
   inherit (config.rose-pine) sources;
@@ -13,28 +14,7 @@ in {
     services.displayManager.sddm.theme = "rose-pine";
 
     environment.systemPackages = [
-      (
+      (pkgs.callPackage ./sddm-rose-pine-pkg.nix {inherit sources;})
-        {
-          stdenvNoCC,
-          libsForQt5,
-        }:
-          stdenvNoCC.mkDerivation {
-            pname = "sddm-rose-pine-theme";
-            version = "1.2";
-            dontBuild = true;
-
-            propagatedUserEnvPkgs = [
-              libsForQt5.qt5.qtgraphicaleffects
-            ];
-
-            src = sources.sddm;
-
-            installPhase = ''
-              mkdir -p $out/share/sddm/themes
-              cp -aR $src $out/share/sddm/themes/rose-pine
-            '';
-          }
-      )
     ];
   };
 }