Documentation and CI fixes

Added

• Document isort's requirement to be run in the same environment as
  the modules which are processed.
• Document VSCode and --lint/-L incompatibility in the README.
• Guard against breaking changes in isort by testing against its main
  branch in the test-future GitHub Workflow.

Fixed

• Fix NixOS builds when pytest-darker calls pylint. Needed to activate
  the virtualenv.
• Allow more time to pass when checking file modification times in a unit test.
  Windows tests on GitHub are sometimes really slow.