refactor(fun): Consistent use of depot.third_party vs. pkgs

In preparation for the solution of b/108, we need to consistently use
`depot.third_party` for packages that are only packed in the TVL depot
and `pkgs` for things that come from nixpkgs.

This commit cleans up a huge chunk of these uses in //fun

Change-Id: I45a7b392a9749fa7859ff5100dcea415bda807c3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2914
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

fun/streamTVL/default.nix

@@ -1,7 +1,7 @@
-{ depot, ... }:
+{ pkgs, ... }:
 
-depot.third_party.writeShellScriptBin "start-tvl-stream" ''
-  env LD_LIBRARY_PATH=/run/opengl-driver/lib/ ${depot.third_party.ffmpeg}/bin/ffmpeg \
+pkgs.writeShellScriptBin "start-tvl-stream" ''
+  env LD_LIBRARY_PATH=/run/opengl-driver/lib/ ${pkgs.ffmpeg-full}/bin/ffmpeg \
        -vsync 0 \
        -hwaccel cuvid \
        -init_hw_device cuda=0 -filter_hw_device 0 \