Change an allowPath call to take a store path again

This looks like a revert of #5844, but is not.

That one was needed because
d90f9d4b99 (diff-0f59bb6f197822ef9f19ceae9624989499d170c84dfdc1f486a8959bb4588cafR85)
changed the type of the argument to `allowPath` from a `StorePath` to a
`Path`. But since
caabc4f648 (diff-0f59bb6f197822ef9f19ceae9624989499d170c84dfdc1f486a8959bb4588cafL100-R92),
it is a `StorePath` again.

I think this is worth changing because we want to be very careful about
`toRealPath` and the evaluator --- ideally the choice of real path does
not affect evaluation at all. So using it fewer times is better.

src/libexpr/primops.cc

@@ -112,7 +112,7 @@ StringMap EvalState::realiseContext(const NixStringContext & context)
     for (auto & outputPath : outputsToCopyAndAllow) {
         /* Add the output of this derivations to the allowed
            paths. */
-        allowPath(store->toRealPath(outputPath));
+        allowPath(outputPath);
     }
 
     return res;