mkarchiso: Do not run _build_iso_base once

archiso/mkarchiso:
Do not run `_build_iso_base` with `_run_once`, as it sets variables which are important to functions that are called
after it.
7 jobs for issues/143 in 43 minutes and 44 seconds (queued for 7 seconds)
latest
Status Name Job ID Coverage
  Check
passed check #30244

00:00:13

 
  Build
passed build_long: [baseline iso] #30247
fast-single-thread

00:19:42

passed build_long: [baseline netboot] #30248
fast-single-thread

00:19:05

passed build_long: [releng iso] #30249
fast-single-thread

00:43:28

passed build_long: [releng netboot] #30250
fast-single-thread

00:40:54

passed build_short: [baseline bootstrap] #30245

00:14:31

passed build_short: [releng bootstrap] #30246

00:14:47