The reason this gets hit is because of the debugger in flakes. Otherwise
you never have a repl in pure mode anyway.
We evaluate the repl-overlay file in impure mode but this doesn't do
what one would initially expect.
Fixes: https://git.lix.systems/lix-project/lix/issues/777
Change-Id: I19b8ed2f5e9ce500b633b13301b42df69ab7deb3