include flowdir in syspath while loading file in Runner API

[madhur-ob]

Context:

The cwd argument of the Runner is for selecting the directory under which the subprocess executes in..

But, it doesn't play a role in loading of the flow file itself -- which is the first step of the lower level API to generate the command that will be executed via subprocess..

aka loading of file --> generate command --> run via subprocess (cwd is only relevant here)

Thus, in the directory structure like this:

├── pipelines/
│   ├── training.py
│   └── common.py
└── tests/
    └── test_sample.py

if training.py has from common import packages
and test_sample.py uses the Runner API as follows:

with Runner("pipelines/training.py", environment="conda").run() as running:
        return running.run.data

the following issue is encountered:

>   from common import packages
E   ModuleNotFoundError: No module named 'common'

This PR adds the flow directory into sys.path and also removes it later...