trivial-builders: add the applyPatches function

applyPatches applies a list of patches to a source directory.

For example to patch nixpkgs you can use:

  applyPatches {
    src = pkgs.path;
    patches = [
      (pkgs.fetchpatch {
        url = "https://github.com/NixOS/nixpkgs/commit/1f770d20550a413e508e081ddc08464e9d08ba3d.patch";
        sha256 = "1nlzx171y3r3jbk0qhvnl711kmdk57jlq4na8f8bs8wz2pbffymr";
      })
    ];
  }