fix source root locations.

These have changed following commit
c3255fe8ec.

See also:

- https://github.com/NixOS/nixpkgs/pull/31004
- https://github.com/NixOS/nixpkgs/pull/31106

pkgs/tools/compression/zstdmt/default.nix

@@ -11,7 +11,9 @@ stdenv.mkDerivation rec {
     owner = "mcmilk";
   };
 
-  sourceRoot = "zstdmt-v${version}-src/unix";
+  setSourceRoot = ''
+    sourceRoot=$(echo */unix)
+  '';
 
   buildInputs = [
     zstd lz4