NixOS Fixes

[blitz]

I'm currently experimenting with generating live patches for NixOS. These are some tiny fixes that make the script work for me.

There are two changes here:

• The first patch fixes the case where kpatch files are installed as read-only (which is the default on NixOS).
• The second patch allows skipping the distro checks, which allows experimenting with new distros. (not necessary)

[blitz]

@joe-lawrence I've dropped the unnecessary change. Can you have a look again?

(Sorry for the long delay, but it was :palm_tree: vacation :palm_tree: time.)

[joe-lawrence]

Thanks, @blitz !