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 3d161f991e

test/units/TEST-74-AUX-UTILS.run.sh

@@ -205,6 +205,13 @@ grep -q "^SocketMode=0644$" "/run/systemd/transient/$UNIT.socket"
 grep -qE "^ExecStart=.*true.*$" "/run/systemd/transient/$UNIT.service"
 systemctl stop "$UNIT.socket" "$UNIT.service" || :
 
+UNIT="socket-no-block-$RANDOM"
+systemd-run --no-block --collect \
+            --unit="$UNIT" \
+            --socket-property=ListenStream=/proc/systemd-run-repro/socket \
+            true
+systemctl stop "$UNIT.socket" "$UNIT.service" || :
+
 : "Job mode"
 systemd-run --job-mode=help
 (! systemd-run --job-mode=foo --scope true)