In restricted mode (and therefore with flakes) `builtins.readFile` may not be the result of `builtins.toFile`,
making it impossible to use a generated lockFile (with or without IFD),
and thereby causing evaluation to fail if `system != builtins.currentSystem` on Hydra
so the jobs are not delegated to eligible build machines that support that system.
This is done in a way that avoids rebuilds.
