Nightly build using PHP 8.4

[sebastianbergmann]

We need a nightly build of all branches (currently: 8.5, 9.6, 10.5, 11.0, main) using the nightly build of the next PHP version (currently PHP 8.4) in order to identify that PHPUnit needs to be updated to a change in the PHP version as soon as possible.

[sebastianbergmann]

According to https://github.com/sebastianbergmann/phpunit/actions?query=event%3Aschedule, the nightly build was only performed for main, but not for 8.5, 9.6, 10.5, and 11.0.

[ghostwriter]

on: schedule event only runs on the last commit of the default branch.

However we may be able to generate matrix of branch names.

- uses: actions/checkout@v4
        with:
          ref: ${matrix.branch}

https://github.com/actions/checkout?tab=readme-ov-file#checkout-a-different-branch