kola/tests/util/update: use correct command name

[pothos]

The switch from coreos-setgoodroot to flatcar-setgoodroot was done for
cmd/kola/updatepayload.go but not yet for kola/tests/util/update.go.
Use flatcar-setgoodroot which is the new name.

How to use

Together with flatcar/scripts#130

Testing done

sudo kola run …run -k -d --board=amd64-usr --platform=qemu --qemu-image=tmp/flatcar_production_qemu_image.img --update-payload=tmp/flatcar_test_update.gz cl.update.payload

--- PASS: cl.update.payload (209.74s)
        cluster.go:117: Created symlink /etc/systemd/system/locksmithd.service → /dev/null.
        update.go:149: Triggering update_engine
        update.go:168: Rebooting test machine
        update.go:149: Triggering update_engine
        update.go:168: Rebooting test machine
PASS, output in _kola_temp/qemu-2021-07-12-1957-2564530