these 2 derivations will be built:
  /nix/store/pccdm5b3nh1xbnn2rfrdhfl6xiq95gry-source.drv
  /nix/store/mafybv5zgmrl29pjrsbcbfmvmxzv3a3m-mpv-youtube-quality-unstable-2020-02-11.drv