Fix fallback to "python" environment when "isolated_build = true" is set

[Unrud]

Skip the section generated by isolated_build = true when collection all environments instead of removing the isolated_build_env afterwards.

Fixes #2474

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
• added yourself to CONTRIBUTORS (preserving alphabetical order)

[gaborbernat]

You'll need to add a test for this.

[Unrud]

I've added a unit test.