bug#39090: Installer fails on install disk built from master

[Gábor Boskovits]

Hello,

I noticed that the install disk created on current master can't
install the system.

This is on:
e4c9ba4da2a6faf80209488d5c086ea0d5c39214
.

I have found a reproducer that might highlight where the problem is.

Steps to reproduce:
1. guix pull
2. build the installer image
guix system disk-image --file-system-type=iso9660 \
  gnu/system/install.scm
3. boot it
4. switch to tty3
5. create a file gexp.scm with this content:
(use-modules
 (gnu packages package-management)
 (guix gexp))

(program-file
 "a"
 (with-extensions
  (list guix)
  #~(#t)))
6. guix build -f gexp.scm

This will fail with as strange error message.
This same problem causes the final installation failure in the installer.

This works fine on the latest installation image.
This works if you run it again after it fails.

Any help on debugging this further would be appreciated.

Best regards,
g_bor
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