check_send_receive_time: Use machine.deploy_type to check "staging".

This brings it in line with other locations (e.g. Sentry) and is less
likely to have accidental false-positives.

puppet/zulip/files/nagios_plugins/zulip_app_frontend/check_send_receive_time

@@ -33,6 +33,7 @@ from django.conf import settings
 
 from zerver.models.realms import get_realm
 from zerver.models.users import get_system_bot
+from zproject.config import get_config
 
 usage = """Usage: send-receive.py [options] [config]
 
@@ -88,7 +89,7 @@ def get_zulips() -> List[Dict[str, Any]]:
 
 internal_realm_id = get_realm(settings.SYSTEM_BOT_REALM).id
 if (
-    "staging" in options.site
+    get_config("machine", "deploy_type") == "staging"
     and settings.NAGIOS_STAGING_SEND_BOT is not None
     and settings.NAGIOS_STAGING_RECEIVE_BOT is not None
 ):