kbuildsycoca: Ignore last modified time when set to UNIX Epoch

On some systems (i.e. Fedora Kinoite), all files in /usr have a last modified timestamp of 0 (UNIX Epoch). In this case, always assume the file as been changed.

We only get the current time once to speed up execution as we don't need to be accurate in this case.

BUG: 442011

See:

• https://bugzilla.redhat.com/show_bug.cgi?id=2021087
• https://discussion.fedoraproject.org/t/kinoite-installation-with-utc-timezone-breaks-kde/34293/8