The previous approach of trying to make both the `override` mechanism from
`mkDerivation` and the `overrideScope'` mechanism from `newScope` work together
resulted in hard to understand code, and there was a bug where once overridden
packages would lose the changes on next override with `packageOverrides`.
It's not ideal still, because Lisps created by `mkDerivation` will lose their
`pkgs` after using `override`.
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