Move ffcheck into a separate package

hosts/marauder/default.nix

@@ -1,9 +1,4 @@
 { self, pkgs, inputs, ... }:
-let
-  ffcheck = pkgs.writeShellScriptBin "ffcheck" ''
-    ${pkgs.ffmpeg}/bin/ffmpeg -v error -stats -hide_banner -i "$1" -c copy -f null -
-  '';
-in
 {
   imports = [
     self.nixosModules.common
@@ -65,7 +60,6 @@ in
 
   environment = {
     systemPackages = [
-      ffcheck
       inputs.shelvacu.packages.x86_64-linux.sm64coopdx
     ] ++ (with pkgs; [
       # Chat clients
@@ -88,6 +82,7 @@ in
       # Multimedia
       vlc
       ffmpeg
+      (callPackage ./ffcheck.nix { })
 
       # Productivity
       obsidian