Motivation for this change

MPV fails with [vapoursynth] Could not initialize VapourSynth scripting., because python can't find vapoursynth module because of wrong path being set in wrapper:

export PYTHONPATH='/nix/store/lr6am5dlv3367d0ng4h04w8hpbhcwbvc-vapoursynth-R49/lib/lib/python3.8/site-packages'${PYTHONPATH:+':'}$PYTHONPATH

(Notice /lib/lib in path)

Can be reproduced with

cd ~/.config/mpv
echo > script.vpy
echo "vf=vapoursynth=~~/script.vpy" >> mpv.conf

And then opening any video

This issue is caused by f93918b
In this commit line

      --prefix PYTHONPATH : "${vapoursynth}/lib/${python3.libPrefix}/site-packages:$PYTHONPATH"

was replaced by

       "--prefix" "PYTHONPATH" ":" "${mpv.vapoursynth}/lib/${mpv.vapoursynth.python3.sitePackages}"

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.