fix: Evaluate flake parent source without evaluating its outputs

This requires that we refer to the `sourceInfo` instead of the
`result`. However, `sourceInfo` does not create a chain of basedir
resolution, so we add that back with `flakeDir`.

tests/functional/flakes/relative-paths.sh

@@ -108,3 +108,24 @@ EOF
 [[ $(nix eval "$rootFlake#z") = 90 ]]
 
 fi
+
+# https://github.com/NixOS/nix/pull/10089#discussion_r2041984987
+# https://github.com/NixOS/nix/issues/13018
+mkdir -p "$TEST_ROOT/issue-13018/example"
+(
+  cd "$TEST_ROOT/issue-13018"
+  git init
+  echo '{ outputs = _: { }; }' >flake.nix
+  cat >example/flake.nix <<EOF
+{
+  inputs.parent.url = ../.;
+  outputs = { parent, ... }: builtins.seq parent { ok = null; };
+}
+EOF
+  git add -N .
+  cd example
+  # Important: the error does not trigger for an in-memory lock!
+  nix flake lock
+  # would fail:
+  nix eval .#ok
+)