make-disk-image.nix: Remove write permissions from /nix/store

Fakeroot seems to always give the owner write bit to any files touched
inside it (presumably to easily simulate the fact that root can still
modify such files). So do an explicit chmod to remove them.

This should finally solve #32242 after the EC2 images are regenerated
with this change.

https://hydra.nixos.org/build/66143116