sysupgrade: create 10_disable_services w/ fixed modified time

sysupgrade: create 10_disable_services w/ fixed modified time

Every time "sysupgrade -b -" runs it would generate a new
(synthetic) "/etc/uci-defaults/10_disable_services" file with
the current time as the modified time.  This unfortunately
creates a non-deterministic tarball, so if you run a cron job
to save your state, you don't have a trivial way of seeing if
it changed or not without unpacking the archive, deleting this
file, and comparing the entire directory tree to the previous
backup.

Fixes: #16145
Fixes: 0ad062a21b ("base-files: sysupgrade: include uci-defaults script disabling services #2")
Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>