Dear Maintainer,

When using the 'chroot' command to install packages within a chroot, I
have observed that systemd-tmpfiles now fails.

Here's a reproducer, using tmux as the package to trigger the issue:

```
$ sudo debootstrap testing debian-testing
[...]

$ sudo chroot debian-testing dpkg-query -W systemd
systemd	262-1

$ sudo chroot debian-testing apt -y install tmux
[...]
Setting up tmux (3.7c-1) ...
/proc/ is not mounted, but required for successful operation of systemd-tmpfiles. Please mount /proc/. Alternatively, consider using the --root= or --image= switches.
dpkg: error processing package tmux (--configure):
 old tmux package postinst maintainer script subprocess failed with exit status 1
 [...]
Errors were encountered while processing:
 tmux
Error: Sub-process /usr/bin/dpkg returned an error code (1)
```

For comparison, here's what I get with trixie:

```
$ sudo debootstrap trixie debian-trixie
[...]

$ sudo chroot debian-trixie dpkg-query -W systemd
systemd	257.13-1~deb13u1

$ sudo chroot debian-trixie apt -y install tmux
[...]
Setting up tmux (3.5a-3) ...
/proc/ is not mounted, but required for successful operation of systemd-tmpfiles. Please mount /proc/. Alternatively, consider using the --root= or --image= switches.
```

We can see that in Trixie, the error message was there, however it
didn't fail.

Something turned it into a hard failure, I don't know if that was
intentional.

Best,

Arnaud