mpv overrides my custom format with `bestvideo+bestaudio/best`
(youtube-dl upstream default). This applies a patch (from upstream) that
adds a magic value to remove the override. Since home-manager’s mpv
module only supports overriding the mpv package in 21.05, it is done in
the overlay.
Fixes#39.
I don’t know if (and doubt that) this is a good solution. I can’t simply
callPackage it, since it does not use a callPackage compatible nix
expression but rather a ready-to-build default.nix. Also, I need the
source in two different files, one of which can’t use nixpkgs fetchers.