clear broken circle cache
clear broken circle cache

caused this error, eg https://app.circleci.com/pipelines/github/snarfed/oauth-dropins/93/workflows/c3e10113-ed70-4b7b-a4de-1d953e7ec3bb/jobs/522 :

```
The virtual environment was not created successfully because ensurepip is not
available.  On Debian/Ubuntu systems, you need to install the python3-venv
package using the following command.

    apt-get install python3-venv

You may need to use sudo with that command.  After installing the python3-venv
package, recreate your virtual environment.

Failing command: ['/root/project/local3/bin/python3', '-Im', 'ensurepip', '--upgrade', '--default-pip']
```

snarfed/oauth-dropins,snarfed/oauth-dropins,snarfed/oauth-dropins