bug#27732: Changing display manager can render system unbootable

[Marius Bakke]

Hello!

When changing display manager in the system configuration and invoking
`guix system reconfigure`, the existing desktop environment will be
killed. If you invoked the command from within said desktop environment,
the reconfigure command will be forcefully terminated.

If you're unlucky, this might happen during `grub-install`. After
logging in on a console and rebooting, this is what I got:

--8<---------------cut here---------------start------------->8---
Welcome to GRUB!

Attempting to decrypt master key...
Enter passphrase for hd1,gpt2 (<uuid>):
Slot 0 opened
error: file `/boot/grub/x86_64-efi/normal.mod' not found.
Entering rescue mode...
grub rescue>
--8<---------------cut here---------------end--------------->8---

It can be recovered by booting by copying /boot/grub/x86_64-efi/* from a
healthy system to the partially installed one.

I'm not sure how to mitigate this, since the DE restart is arguably a
feature. A command analogous to `nixos-rebuild boot` might be of aid for
those of us aware of this problem.

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