[Core] Task fails when run section is to long

[Michaelvll]

#3394 removed the rsync for the script for job submission, which makes a long user yaml causing the following error:

OSError: [Errno 7] Argument list too long: '/bin/sh'

To reproduce:

1. create a YAML: test.yaml

resources:
  cpus: 2

run: |
  echo hi

2. Make the run section long: for i in `seq 1 100000`; do echo " echo $i" >> test.yaml; done
3. sky launch -c test-long-user-run test.yaml