these 2 derivations will be built:
  /nix/store/9zwsw9z5s4m8hmg617j4jbaigkfvq7qk-nix-source-patched-source.drv
  /nix/store/j6155hjjg3j3rgwrj84zhvyzb2a50rvj-equal-contents-trivial-patch-does-not-change-source-contents.drv