kbx: Fix regression in compression trigger from July 18

* kbx/keybox-update.c (keybox_compress): Change condition back.
Also use make_timestamp for CUT_TIME.

Note that the original change was not backported to 2.2.

• Fixes-commit: 824ca6f042dc69edaf67bf9d4e875be75babab00
• Signed-off-by: Werner Koch <wk@gnupg.org>