librsvg: fix cross compile

- use docutils for buildPlatform
- disable the postFixup step when withIntrospection is disabled

pkgs/development/libraries/librsvg/default.nix

@@ -17,7 +17,7 @@
 , rust
 , cargo
 , gi-docgen
-, python3
+, python3Packages
 , gnome
 , vala
 , withIntrospection ? stdenv.hostPlatform == stdenv.buildPlatform
@@ -49,7 +49,7 @@ stdenv.mkDerivation rec {
     pkg-config
     rustc
     cargo
-    python3.pkgs.docutils
+    python3Packages.docutils
     vala
     rustPlatform.cargoSetupHook
   ] ++ lib.optionals withIntrospection [
@@ -125,7 +125,7 @@ stdenv.mkDerivation rec {
     mv $GDK_PIXBUF/loaders.cache.tmp $GDK_PIXBUF/loaders.cache
   '';
 
-  postFixup = ''
+  postFixup = lib.optionalString withIntrospection ''
     # Cannot be in postInstall, otherwise _multioutDocs hook in preFixup will move right back.
     moveToOutput "share/doc" "$devdoc"
   '';