The become_user pattern with sudo doesn't reliably connect to the
user's D-Bus session, causing systemd user timers (docker-prune,
runner-healthcheck) to silently fail to enable. This led to stale
Docker images accumulating and filling the disk on CI runners.
Switch all systemctl --user invocations to use runuser with explicit
XDG_RUNTIME_DIR and DBUS_SESSION_BUS_ADDRESS environment variables,
which correctly connects to the user's systemd session.
Affected tasks: prune timer, healthcheck timer, gitea-runner service,
rootless Docker daemon start/enable/restart, runner service restart.
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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>
