You don't. Those scripts will just fail forcing you to rewrite them. This is why some people trying to create new packages often complain, because they need to

You don't. Those scripts will just fail forcing you to rewrite them. This is why some people trying to create new packages often complain, because they need to patch up original build for given application to not do those things. There are still ways that package will not be fully reproducible, for example if it uses rand() during build, Nix doesn't patch that, but stuff like that is fortunately not common.