Added ffmpeg to opencv

flake.nix

@@ -12,7 +12,12 @@
 
         pythonPackages = pkgs.python312Packages;
 
-        custom-opencv4 = pythonPackages.opencv4.override { enableGtk3 = true; };
+        custom-opencv4 = pythonPackages.opencv4.override {
+          enableGtk3 = true;
+          enableFfmpeg =
+            true; # here is how to add ffmpeg and other compilation flags
+          ffmpeg = pkgs.ffmpeg-full;
+        };
 
         lapx = pkgs.python3Packages.buildPythonPackage rec {
           version = "0.5.11";