Add-on store: only make directories if should write to disk

[seanbudd]

Link to issue number:

Fix up of #14955
Raised in #14912 (comment)

Summary of the issue:

Caching directories are still created when NVDA should not write to disk

Description of user facing changes

Caching directories are no longer created when NVDA should not write to disk

Description of development approach

Caching directories are no longer created when NVDA should not write to disk

Testing strategy:

Refer to #14955

Known issues with pull request:

N/a

Change log entries:

N/a

Code Review Checklist:

• Pull Request description:
  • description is up to date
  • change log entries
• Testing:
  • Unit tests
  • System (end to end) tests
  • Manual testing
• API is compatible with existing add-ons.
• Documentation:
  • User Documentation
  • Developer / Technical Documentation
  • Context sensitive help for GUI changes
• UX of all users considered:
  • Speech
  • Braille
  • Low Vision
  • Different web browsers
  • Localization in other languages / culture than English
• Security precautions taken.