obs-studio: enable AV1 encoding

OBS Studio 27.2 comes with support for AV1 video encoding, but AV1
support is not enabled in our default ffmpeg build. Switch OBS over to
use ffmpeg-full, which comes with the optimized svt-av1 encoder.

pkgs/top-level/all-packages.nix

@@ -27865,7 +27865,9 @@ with pkgs;
 
   oberon-risc-emu = callPackage ../applications/emulators/oberon-risc-emu { };
 
-  obs-studio = libsForQt5.callPackage ../applications/video/obs-studio {};
+  obs-studio = libsForQt5.callPackage ../applications/video/obs-studio {
+    ffmpeg_4 = ffmpeg-full;
+  };
   obs-studio-plugins = recurseIntoAttrs (callPackage ../applications/video/obs-studio/plugins {});
   wrapOBS = callPackage ../applications/video/obs-studio/wrapper.nix {};