wl-screenrec: pin ffmpeg_6

From <https://github.com/russelltg/wl-screenrec/blob/v0.1.3/README.md#from-source-using-cargo>:

> Install ffmpeg 6 or later, which is a required dependency. ffmpeg
> 5 may work, but is untested (open an issue or PR if you test with
> ffmpeg 5 so I can update these docs on if it works or not)

I guess it worked…

7.x will need a bump to the ffmpeg-sys-next crate, but see
<russelltg/wl-screenrec#77>.

pkgs/top-level/all-packages.nix

@@ -4666,7 +4666,7 @@ with pkgs;
   wl-mirror = callPackage ../tools/wayland/wl-mirror { };
 
   wl-screenrec = callPackage ../tools/wayland/wl-screenrec {
-    ffmpeg = ffmpeg_5;
+    ffmpeg = ffmpeg_6;
   };
 
   wleave = callPackage ../tools/wayland/wleave { };