disable substitutes for grub config and bootloader installation script

[pkill9]

rolling back the system shows it's trying to get substitutes for grub.cfg.drv 
and install-bootloader.scm.drv:
```
itsme@antelope ~> sudo guix system roll-back
Password: 
substitute: updating substitutes from 'https://berlin.guixsd.org'... 100.0%
substitute: updating substitutes from 'https://mirror.hydra.gnu.org'... 100.0%
substitute: updating substitutes from 'https://ci.guix.gnu.org'... 100.0%
The following derivation will be built:
   /gnu/store/b90j6p6qpzvs5x884hhy9dc3zvv6jhcs-grub.cfg.drv
building /gnu/store/b90j6p6qpzvs5x884hhy9dc3zvv6jhcs-grub.cfg.drv...
substitute: updating substitutes from 'https://berlin.guixsd.org'... 100.0%
substitute: updating substitutes from 'https://mirror.hydra.gnu.org'... 100.0%
substitute: updating substitutes from 'https://ci.guix.gnu.org'... 100.0%
building 
/gnu/store/m7lcl11s38ks2hapm1j7s2z9szig3zhs-install-bootloader.scm.drv...
switched from generation 46 to 45
```

This is not needed, unless I am missing something?