CI fails when new version of virtualenv is used

[michalpristas]

Followup issue to this PR #16235

When CI fetches latest virtualenv (20.0.0 released 10 Feb)
It fails on

Generated fields.yml for filebeat to /Users/travis/gopath/src/github.com/elastic/beats/filebeat/build/fields/fields.all.yml
Traceback (most recent call last):
  File "/Users/travis/gopath/src/github.com/elastic/beats/build/ve/darwin/bin/pip", line 6, in <module>
    from pip._internal.cli.main import main
ImportError: No module named main
Error: running "/Users/travis/gopath/src/github.com/elastic/beats/build/ve/darwin/bin/pip install --quiet -Ur /Users/travis/gopath/src/github.com/elastic/beats/libbeat/tests/system/requirements.txt" failed with exit code 1
make: *** [update] Error 1

[jsoriano]

Closing this as we are not using virtualenv since #14798.