Bug#1040899: debian-cd: "Failed to mount /target/proc" during GRUB installation

Package: debian-cd
Severity: important

Hi,

installing Debian 13 testing using today's image [1] always fails at the 
boot loader installation stage with the error "Failed to mount 
/target/proc". After this message is shown, GRUB is left uninstalled and 
it is not possible the boot the system.

Running `mount` in a shell shows that /target/proc is already mounted.

This error occurs regardless of the installation method (tested with 
graphical, tui, expert install).

Choices: All default answers except:

* Language: C
* Zone: Europe/Germany
* Software selection: SSH server + standard sys utils

[1] 
https://cdimage.debian.org/cdimage/daily-builds/daily/20230712-1/amd64/iso-cd/debian-testing-amd64-netinst.iso