bug#47442: guix system delete-generations does not use bootloader configuration

[raid5atemyhomework]

Hello,

I use a coreboot that does not have a VGA option rom, which means Grub can't 
use `gfxterm`, so I have this setting in my `operating-system`:

    (bootloader-configuration
      ; ...
      (terminal-outputs '(console)))

This lets me see a boot menu at startup even without the VGA option rom.

However, recently when I did a `guix system delete-generations` to remove some 
old configurations, I saw to my dismay that the `/boot/grub/grub.cfg` got 
reverted to `terminal_output gfxterm`.

Creating a pointless whitespace change to a daemon configuration file and `guix 
system reconfigure` got this fixed.  This workaround is undesirable since it 
makes deleting old generations a multi-step, non-atomic process.


The expected behavior is that whatever settings were used in my latest 
`bootloader-configuration` should not be touched by `guix system 
delete-generations`.
The actual behavior is `guix system delete-generations` uses the default 
configuration.

Looking at the code, it seems to be an issue with `guix/scripts/system.scm`'s 
`reinstall-bootloader` function:

```scheme
         ;; Use the detected bootloader with default configuration.
         ;; It will be enough to allow the system to boot.
         (bootloader-config (bootloader-configuration
                             (bootloader bootloader)))
```

While "it will be enough to allow the system to boot", it is not enough for me, 
on my specific system, to get a convenient boot menu where I can, you know, 
select older systems (absolutely vital due to the brittleness of Shepherd).  On 
my system, using `terminal_output gfxterm` leads to an "invisible" boot menu, 
making Guix's vaunted ability to have multiple bootable systems fairly useless 
--- I can't be sure what system I end up booting into since I can't see the 
boot menu.  I think on other systems as well that need other terminal outputs 
(such as some sort of `serial`) this will be a problem as well.

A possible solution would be to check if the current system has a `provenance`, 
and if so, to read in the file and look for the `configuration` field.  Then 
read *that* file and extract its `bootloader` field, and to only fall back to a 
default configuration if that attempt fails.  See 
https://lists.gnu.org/archive/html/guix-devel/2021-03/msg00294.html for similar 
idea.  Would this be an acceptable solution?


Thanks
raid5atemyhomework