Bug#998242: cron-failure at .service delivery fails due to DynamicUser with exim

[Yuri D'Elia]

Package: systemd-cron
Version: 1.5.17-3
Severity: important

cron-failure@ is using DynamicUser=yes, however this causes a silent
delivery failure when using exim4:

systemd[1]: Starting cron-failure at cron-monthly.service...
mail_on_failure[328561]: 2021-11-01 14:11:23 1mhX5v-001NTN-LU Failed to create spool file /var/spool/exim4//input//1mhX5v-001NTN-LU-D: Permission denied
mail_on_failure[328561]: 2021-11-01 14:11:23 1mhX5v-001NTN-LU Failed to create spool file /var/spool/exim4//input//1mhX5v-001NTN-LU-D: Permission denied
mail_on_failure[328561]: 2021-11-01 14:11:23 1mhX5v-001NTN-LU Failed to create spool file /var/spool/exim4//input//1mhX5v-001NTN-LU-D: Permission denied
systemd[1]: cron-failure at cron-monthly.service: Deactivated successfully.
systemd[1]: Finished cron-failure at cron-monthly.service.

Combined with #992649, this makes systemd-cron silently broken on a
debian configuration with the default mta.

-- Package-specific info:
-- output of systemd-delta

-- System Information:
Debian Release: bookworm/sid
  APT prefers unstable
  APT policy: (900, 'unstable'), (800, 'experimental'), (500, 'unstable-debug')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 5.14.0-3-amd64 (SMP w/8 CPU threads)
Kernel taint flags: TAINT_PROPRIETARY_MODULE, TAINT_WARN, TAINT_OOT_MODULE, TAINT_UNSIGNED_MODULE
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8), LANGUAGE not set
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages systemd-cron depends on:
ii  libc6         2.32-4
ii  python3       3.9.2-3
ii  systemd-sysv  249.5-1

Versions of packages systemd-cron recommends:
ii  exim4-daemon-light [mail-transport-agent]  4.95-2

systemd-cron suggests no packages.