fetchInfo.path cannot point to derivation

[DavHau]

example override:

{
  floco.pdefs.prettier."2.8.7".fetchInfo = l.mkForce {
    path = "${config.nodejs-floco.source}";
  };
}

This leads to an error like:

error: access to absolute path '/nix/store/kcb6fw0g6swnfyacb0y1fn2dzlmdzhfc-source/package.json' is forbidden in pure eval mode (use '--impure' to override)

I'm guessing that some usage of builtins.unsafeDiscardStringContext might be the cause.