Add timestamp to version to avoid hash being interpreted as downgrade

[mortenlj]

When trying to update from 2021-02-16-efb9783 to 2021-02-16-0cea242, dpkg on linux complains that this is a downgrade. The reason is the hashes are the only differing part, and they are sorted as version numbers. To avoid this, we should include a timestamp in the version string, with at least hours and minutes.