Only build 64bit NVDA (#18924)
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### Link to issue number:
Part of #16304
### Summary of the issue:
We are migrating to 64bit NVDA in 2026.1
### Description of user facing changes:
Switch alpha builds to 64bit
### Description of developer facing changes:
### Description of development approach:
### Testing strategy:
### Known issues with pull request:
Blocked by:
- A stable installer
### Code Review Checklist:
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- [x] Documentation:
- Change log entry
- User Documentation
- Developer / Technical Documentation
- Context sensitive help for GUI changes
- [x] Testing:
- Unit tests
- System (end to end) tests
- Manual testing
- [x] UX of all users considered:
- Speech
- Braille
- Low Vision
- Different web browsers
- Localization in other languages / culture than English
- [x] API is compatible with existing add-ons.
- [x] Security precautions taken.
---------
Co-authored-by: Tianze <tianzeshi_study@outlook.com>
Co-authored-by: Michael Curran <mick@nvaccess.org>
