ci: sync time domains

Some parts of the logs can use $CI_JOB_STARTED_AT, while other parts use
the uptime (time since kernel boot) instead.

Printing both here allows syncing them and being able to tell how much
time actually elapsed between log lines.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/39395>

.gitlab-ci/common/init-stage2.sh

@@ -54,6 +54,12 @@ echo
 
 section_switch init_stage2 "Pre-testing hardware setup"
 
+job_time=$(get_job_seconds)
+uptime=$(cut -d ' ' -f1 /proc/uptime)
+echo "$(get_current_minsec) after job start == $uptime sec after kernel boot time"
+printf -v uptime_rounded "%.0f" "$uptime"
+echo "Kernel boot occurred $((job_time-uptime_rounded)) seconds after job start"
+
 set -ex
 
 # Set up any devices required by the jobs