file: Only determine the local timezone once when generating date string

We can re-use the local time zone that was created for the the
file_date_time for all following date times that also use the local time
zone. This also helps with the glib time zone performance regression
by reducing the time zones created for each file from four to one.

Related: GNOME/glib#2204
GNOME/nautilus!601
2 jobs for date-string-cache in 8 minutes and 23 seconds (queued for 2 seconds)
latest
Status Name Job ID Coverage
  Test
passed flatpak devel #910124
flatpak

00:08:21

passed style check #910125

00:01:17