repl: fix repl-overlays in pure eval mode

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

tests/functional/repl_characterization/data/extra_data/repl-overlay-trivial.nix

@@ -0,0 +1,3 @@
+info: final: prev: {
+  foo = 2;
+}