fix `deepspeed --venv_script` (#7469)
currently passing `deepspeed ... --venv_script foo.sh` ends up with a
pdsh cmd like:
```
pdsh -S -f 1024 -w 10.4.11.15,10.4.10.1 source foo.sh export NCCL_NET_PLUGIN=blah; ...
```
you can see, `;` is missing before exports start, so the first export is
ignored.
It should be:
```
pdsh -S -f 1024 -w 10.4.11.15,10.4.10.1 source foo.sh; export NCCL_NET_PLUGIN=blah; ...
```
This PR is fixing it.
