GitHub fetcher: Ignore treeHash attribute for forward compatibility

See https://github.com/NixOS/nix/pull/10197.

src/libfetchers/github.cc

@@ -109,6 +109,7 @@ struct GitArchiveInputScheme : InputScheme
             "narHash",
             "lastModified",
             "host",
+            "treeHash",
         };
     }