mkarchiso: replace external date command with printf

printf is a bash builtin, so by using it an external command can be avoided.
Due to the differences between date(1) and strftime(3), the time zone output will not contain a colon anymore. Fortunately, that is still a supported format according to https://en.wikipedia.org/wiki/ISO_8601#Time_offsets_from_UTC .
8 jobs for date in 49 minutes and 53 seconds (queued for 6 seconds)
latest
Status Name Job ID Coverage
  Check
passed check #33570

00:00:15

 
  Build
passed build_long: [baseline iso] #33579
fast-single-thread

00:15:22

passed build_long: [baseline netboot] #33574
fast-single-thread

00:17:05

passed build_long: [releng iso] #33575
fast-single-thread

00:34:13

passed build_long: [releng netboot] #33576
fast-single-thread

00:32:06

passed build_short: [baseline bootstrap] #33571

00:12:15

passed build_short: [releng bootstrap] #33572

00:12:54

failed build_long: [baseline iso] #33573
fast-single-thread

00:48:53