Building Python SDK harness fails to build without virtualenv pre-installed

Hi all,

I tried building the python sdk harness according to [https://beam.apache.org/documentation/runners/flink/](https://beam.apache.org/documentation/runners/flink/) and it failed for me when I didn't have virtualenv installed (and succeeded after I did install it for the system python env).

Specific error message:
```

> Task :sdks:python:setupVirtualenv FAILED

```

and below
```

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':sdks:python:setupVirtualenv'.
>
A problem occurred starting process 'command 'virtualenv''

```

Not sure if this can be fixed with gradle or if it's easier to just publish the docker image so I don't need to build it myself. Alternatively the virtualenv prerequisite could also be added to the guide of course.

Imported from Jira [BEAM-7593](https://issues.apache.org/jira/browse/BEAM-7593). Original Jira may contain additional context.
Reported by: jjbankert.