fix(sct.py): list-resources for docker failed for no `SSH_AGENT_PID`
fix(sct.py): list-resources for docker failed for no `SSH_AGENT_PID`

Running sct.py list/clean-resources from commandline (not inside hydra),
yielded the following error, only for a warning print.
```
  File "/home/fruch/Projects/scylla-cluster-tests/sdcm/utils/common.py"
    SSHAgent.start(verbose=verbose)
  File "/home/fruch/Projects/scylla-cluster-tests/sdcm/utils/ssh_agent.py"
    os.environ["SSH_AUTH_SOCK"], os.environ["SSH_AGENT_PID"])
  File "/home/fruch/.pyenv/versions/3.10.0/lib/python3.10/os.py"
    raise KeyError(key) from None
KeyError: 'SSH_AGENT_PID'
```

scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests