Include Legacy Widget block files in the plugin build

[Mamaduka]

Description

Updates plugin ZIP creation to include the Legacy Widget block files.

Fixes #32785.
Fixes #32795.

How has this been tested?

1. Run bin/build-plugin-zip.sh in plugin directory.
2. Make sure the widgets package has a block directory with structure:

blocks/
  legecy-widget/block.json
  legacy-widget.php

Types of changes

Bugfix

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[ockham]

I tested this with a fresh WP install. I installed GB 10.9 RC1 and verified that the error messages seen in the issues linked in the PR desc were present.

I then removed it and installed the plugin build from this PR instead. This time, I didn't get the same error messages 👍

Also noting that the issue isn't present in 10.8.2, so we don't have to backport to the 10.8 branch.

[Mamaduka]

Thanks for testing, @ockham.

[ockham]

Cherry-picked to release/10.9.