wic: honor the SOURCE_DATE_EPOCH in case of updated fstab

In case user requested to build a binary repeatable package,
it's required to honor the SOURCE_DATE_EPOCH environment
variable. So forcefully set mtime inside all the routines
which modify fstab in case it is updated.

Signed-off-by: Sergei Zhmylev <s.zhmylev@yadro.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>