[testgen] Support year override

The testgen golden tests fail every new year because the copyright lines
do not match. This change re-enables the testgen build and supports
--year-override, ensuring all the output files remain the same with
their initial creation year of 2024.

Bug: 318541594
Change-Id: I46af4701ed195308985e918e547a4bff3dc7e10e
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1181592
Reviewed-by: Oliver Newman <olivernewman@google.com>
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>
Reviewed-by: Brian Bosak <bbosak@google.com>
Fuchsia-Auto-Submit: Christopher Johnson <crjohns@google.com>
(cherry picked from commit ccf560db629b213f78be125976dcb5469f721935)