bug#44898: [wishlist] Make the GRUB installation procedure smarter

[guixuser6392]

Every time the operating system is instantiated the GRUB boot-loader is 
inexplicitly re-installed. This behaviour leads to unsolicited changes to the 
user's boot configuration on UEFI systems; and leads to unnecessary write 
operations on the ESP, and/or the MBR, which, in case they are abruptly aborted 
during the building of the install-bootloader derivation, can leave the system 
in an unbootable state. Futhermore, frequent writes to the platform's NV-RAM 
may negatively impact its lifespan.

NixOS stores the GRUB derivation as well as boot parameters in a state file, 
and only re-installs the boot-loader if the computed state is different from 
the stored state: 
https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/system/boot/loader/grub/install-grub.pl#L626.
 However, unless I am mistaken, that alone will not prevent GRUB from altering 
the user's boot configuration in a scenario, in which we want GRUB to be 
re-installed because it has received an update.

To summarize, I think that consecutive grub-install invocations should not be 
allowed to modify the user's boot configuration on UEFI systems, and that we 
should teach Guix to only re-install the boot-loader if the boot parameters has 
been changed and/or the GRUB package has been upgraded.

Although other boot-loaders are not in the scope of this wish, brining their 
installation procedures to the same high standard would be beneficial, too.