[testing workflows] Move PHP tests with WP nighly back into ci.yml

[adimoldovan]

Submission Review Guidelines:

• I have followed the WooCommerce Contributing Guidelines and the WordPress Coding Standards.
• I have checked to ensure there aren't other open Pull Requests for the same update/change.
• I have reviewed my code for security best practices.

Changes proposed in this Pull Request:

With optional tests now supported in ci.yml we can move the WP nightly back. They were removed by #45096 and then added to a cron workflow with #45260.

How to test the changes in this Pull Request:

Check CI, make sure PHP WP: nightly are running for @woocommerce/plugin-woocommerce. The job should be optional.

[github-actions]

Test using WordPress Playground

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Test this pull request with WordPress Playground.

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[github-actions]

Hi , @woocommerce/vortex

Apart from reviewing the code changes, please make sure to review the testing instructions as well.

You can follow this guide to find out what good testing instructions should look like:
https://github.com/woocommerce/woocommerce/wiki/Writing-high-quality-testing-instructions

[rrennick]

Nice work