Prevent unscheduled release publication

Prevent unscheduled release publication

The "publish" job has no dependencies on other jobs, so nothing prevents
it from being accidentally started before the scheduled publication
date.  Although publication still requires confirmation via an SSH
connection to a dedicated, locked-down runner, performing that action
prematurely may have drastic consequences.  Therefore, it is worth
implementing additional safeguards.

Add an extra check to the "publish" job to ensure it can only be run on
the scheduled publication day.  In exceptional circumstances, this check
can be overridden by setting the FORCE_PUBLICATION CI variable to any
non-empty value.