Test building an ISO with a fake system time set in 2018

Originally created by @intrigeri on #12351 (Redmine)

… in order to check if it matches an ISO built without faking the time, i.e. whether the build year variation breaks reproducibility.

Performing this test earlier in 2017 is too painful due to various indices and keys having expired.

Use the dateoffset build option to do this.

Edited May 15, 2020 by intrigeri

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