Allow substitutes for Plymouth theme ⚡

When using `runCommandLocal`, clients are required to download imagemagick
and the source files, even if the theme is already available from a binary
cache.

modules/plymouth/default.nix

@@ -4,7 +4,7 @@ with lib;
 with config.lib.stylix.colors;
 
 let
-  theme = pkgs.runCommandLocal "plymouth-theme" { } ''
+  theme = pkgs.runCommand "stylix-plymouth" { } ''
     themeDir="$out/share/plymouth/themes/stylix"
     mkdir -p $themeDir