fix(agent): make package update checks work as the sandboxed agent user

The agent runs as the beszel user under ProtectSystem=strict, so package
managers that write temp, cache or log files failed with only
"exit status 1" in the log.

- Include the command's stderr in check errors.
- dnf5 (Fedora 41+): point XDG_STATE_HOME and XDG_CACHE_HOME into the
  agent's data dir. They default to $HOME, which is /nonexistent for
  the beszel user. With -C, dnf5 still reads the system metadata cache.
- zypper: keep the solv cache in the data dir with --solv-cache-dir,
  since the agent user cannot rebuild it in /var/cache/zypp.
- pacman: read the system sync databases with pacman -Qu instead of
  syncing a private copy with checkupdates. This drops the
  pacman-contrib and fakeroot dependencies and pacman's download
  sandbox, which fails in containers without Landlock. Like the other
  managers, it never refreshes the databases itself.
- install-agent.sh, Debian unit: add PrivateTmp=yes. The script adds it
  to existing units too when re-run. dnf4 (Rocky, Alma) and zypper keep
  temporary files in /var/tmp, which ProtectSystem=strict makes
  read-only. dnf4 hardcodes /var/tmp, so the agent cannot work around
  it. apt does not need it; the Debian unit gets it so both units stay
  the same.

agent/test-data/package_updates/pacman_checkupdates.txt

@@ -1,4 +0,0 @@
-libpcap 1.10.7-1 -> 1.11.0-1
-libsecret 0.21.7-1 -> 0.21.8.2-1
-libtirpc 1.3.7-1 -> 1.3.8-1
-tzdata 2026c-1 -> 2026d-1