Widgets: Retrieve latest widgets before loading sidebars

[walbo]

Description

Backport https://core.trac.wordpress.org/ticket/53489 that was fixed in core.

This fixes issues where sidebars would be unexpectedly missing from the new widgets screen. Running retrieve_widgets syncs sidebars that were registered after the last theme switch.

How has this been tested?

See testing in core ticket.

Types of changes

Bug fix

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).

[github-actions]

👋 Thanks for your first Pull Request and for helping build the future of Gutenberg and WordPress, @walbo! In case you missed it, we'd love to have you join us in our Slack community, where we hold regularly weekly meetings open to anyone to coordinate with each other.

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[talldan]

Thanks!