Use mksquashfs' SOURCE_DATE_EPOCH support

[zeha]

mksquashfs -help-section environment has to say:

SOURCE_DATE_EPOCH If set, this is used as the filesystem creation timestamp. Also any file timestamps which are after SOURCE_DATE_EPOCH will be clamped to SOURCE_DATE_EPOCH. See https://reproducible-builds.org/docs/source-date-epoch/ for more information

Ideally we can drop our cap_timestamps hack. grml-live/minifai needs to export SOURCE_DATE_EPOCH for this to work.