Error out if trying to build an ISO with a system time before SOURCE_DATE_EPOCH
Doing so would break not only build reproducibility, but probably a number of more or less implicit assumptions our hacks for #5630 (closed) make.
Feature Branch: feature/12352-fail-builds-before-source_date_epoch
Parent Task: #5630 (closed)
Original created by @intrigeri on 12352 (Redmine)