workflows: enforce RFC166 formatting for mpvScripts

.github/workflows/check-nix-format.yml

···
         # Each environment variable beginning with NIX_FMT_PATHS_ is a list of
         # paths to check with nixfmt.
         env:
+          NIX_FMT_PATHS_MPVSCRIPTS: pkgs/applications/video/mpv/scripts
           # Format paths related to the Nixpkgs CUDA ecosystem.
           NIX_FMT_PATHS_CUDA: |
             pkgs/development/cuda-modules