run: honor --no-block for trigger units

run: honor --no-block for trigger units

Reproducer:
  unit=run-nb-trigger-$(date +%s)
  systemd-run --no-block --collect --unit="$unit" \
      --socket-property=ListenStream=/proc/systemd-run-repro/socket \
      /usr/bin/true
  echo $?

Before, systemd-run still waited for the trigger unit job and
propagated the socket start failure. With --no-block it should only
verify and enqueue the request, as the service path already does.

Follow-up for 3d161f991e16369aa59f447eb4cdb90af33261c8