mkDerivation: Fix errors from #27365

`nix-build pkgs/top-level/release.nix -A tarball` now succeeds.

`configureFlags = null` lead to a type error, and one overrideDrv
needed to be converted to to append a configureFlags list instead of
string due to the normalization.

Thanks @vcunat for alerting me to the issues---sorry I did not catch
them before merging my own PR.