Harden Panama doctor probes

tests/quickshell/fixtures/doctor/bin/systemctl

@@ -2,6 +2,10 @@
 
 set -euo pipefail
 
+if [[ -n ${PANAMA_DOCTOR_FIXTURE_PROBE_SECRET+x} ]]; then
+    exit 97
+fi
+
 service="${4:-}"
 case ",${PANAMA_DOCTOR_FIXTURE_STOPPED:-}," in
     *",$service,"*) exit 3 ;;