Don't initialize runner env unless needed #14

PythonRunner was not needed in current run, yet got initialized.

$ alice -v -i ci-examples/full.yaml pkg

[Alice] Available runners: python|pypi
[Alice] Configured globals: { ... }
[Alice] Configuring runner: python
[Alice] Initializing runner: python
[PythonRunner] Initializing
[PythonRunner] Found virtualenv at /workspaces/alice/packages/venv
[Alice] Parsed jobs: env, lint, pkg
[Alice] Begin pipeline steps
[Alice] Initializing runner: pypi
[PyPiRunner] Initializing
{'name': 'pkg', 'type': 'pypi', 'workdir': '.', 'upload': False, 'repo': {'uri': 'example.com', 'username': 'asdf', 'password': {'from_env': 'PYPIPASS'}}, 'packages': ['alice-ci']}
[Alice][Step] pkg: SUCCESS

The function update_config needs to be revised.