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

… 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.

Original created by @intrigeri on 12351 (Redmine)

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