Due to a mistake I did wrt. shell quoting in
commit:6fcb2323, we were actually passing "--id
'1490015021'" (with the single quotes), which is an invalid ID. So isohybrid
was aborting. As a result isohybrid stopped creating non-reproducible ISOs,
but the resulting ISO images were not hybrid'ed.

Let's not bother passing $SOURCE_DATE_EPOCH, and just pass a fixed value.