Fix global configuration fallback in Jenkins runs

[daneah]

Fixes #1428

When running in Jenkins and using setup.cfg, tox would not fall back to the [tox:tox] configuration section properly. This was due to how the fallback section name was calculated under this specific combination of characteristics.

Thanks for contributing a pull request!

If you are contributing for the first time or provide a trivial fix don't worry too
much about the checklist - we will help you get started.

Contribution checklist:

(also see CONTRIBUTING.rst for details)

• wrote descriptive pull request text
• added/updated test(s)
• updated/extended the documentation
• added relevant issue keyword
  in message body
• added news fragment in changelog folder
  • fragment name: <issue number>.<type>.rst for example (588.bugfix.rst)
  • <type> is must be one of bugfix, feature, deprecation,breaking, doc, misc
  • if PR has no issue: consider creating one first or change it to the PR number after creating the PR
  • "sign" fragment with "by :user:<your username>"
  • please use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files - by :user:superuser."
  • also see examples
• added yourself to CONTRIBUTORS (preserving alphabetical order)

[asottile]

awesome! that was easy

[gaborbernat]

The CI failure is not related to this PR, so will accept. Thanks for your contribution!