The harness runs as root in its container, so test-runs and the serial
logs QEMU writes there belong to root. The runner's user could then
neither record the pull request number beside them nor read
install-serial.log into the artifact (EACCES at zip time), which failed
the job after a passing install and boot. A chown -R step after the
container step, run even on failure, fixes both. The install loop now
prints a line every 30 s and still screenshots every 2 min.