/usr/bin/grub-editenv: error: invalid environment block.

Upgrading GRUB on a ppc64le machine (8335-GCA (IBM Power System S822LC))

    2021-03-15 08:20:23 upgrade grub-common:ppc64el 2.04-1ubuntu26.8 2.04-1ubuntu26.11

`grub-initrd-fallback.service` failed to restart.

```
$ sudo journalctl -xe
[…]
Mar 15 08:20:30 s822lc grub-editenv[2481630]: /usr/bin/grub-editenv: error: invalid environment block.
Mar 15 08:20:30 s822lc systemd[1]: grub-initrd-fallback.service: Main process exited, code=exited, status=1/FAILURE
-- Subject: Unit process exited
-- Defined-By: systemd
-- Support: http://www.ubuntu.com/support
--
-- An ExecStart= process belonging to unit grub-initrd-fallback.service has exited.
--
-- The process' exit code is 'exited' and its exit status is 1.
Mar 15 08:20:30 s822lc systemd[1]: grub-initrd-fallback.service: Failed with result 'exit-code'.
-- Subject: Unit failed
-- Defined-By: systemd
-- Support: http://www.ubuntu.com/support
--
-- The unit grub-initrd-fallback.service has entered the 'failed' state with result 'exit-code'.
Mar 15 08:20:30 s822lc systemd[1]: Failed to start GRUB failed boot detection.
-- Subject: A start job for unit grub-initrd-fallback.service has failed
```