Calling run with external-env should not check for project files

[AlbertDeFusco]

Since the use of --external-env completely ignores any environment defined in the project we don't need to fail if no environment.yml or conda-project.yml file is found

> mkdir empty
> cd empty
> conda-project run --external-env base python -c 'import sys;print(sys.executable)'
CondaProjectError: No conda environment.yml or environment.yaml file was found.

This should instead return the path of the Python executable in your base conda env.