refine job submit cmake list, resolve python import error

[huahuiyang]

python job_submit.py <webUI port (8080)>

would throw some proto class not found error, such as:

ImportError: cannot import name label_pb2
ImportError: cannot import name label_selector_pb2

this pr refine the cmake list to resolve that issue

[huahuiyang]

@ICGog @ms705

[ICGog]

Thanks for the change! I've merged it via gerrit: https://review.gerrithub.io/c/camsas/firmament/+/418906