"server up" check in perf.yml is in the wrong place

https://github.com/mongodb/mongo/blob/a4b34582b6fd71785f8be653409293788476f03f/etc/perf.yml#L59-L61

that line of the script needs to be done in a separate command. as written, it is useless because it runs in a command with "background: true" so it won't actually block the next command in the task.