mkosi: Stop using git commit timestamps for package releases

mkosi: Stop using git commit timestamps for package releases

This prevents bisecting to figure out which commit broke something
as when going backwards the git commit timestamp will be older meaning
package managers will refuse to upgrade to the "older" version. Let's
make sure the release is always newer by using the current date unless
$SOURCE_DATE_EPOCH is set.

(cherry picked from commit caf5eb586afa1a71ed04a3442585536930b150c2)