chore: fix php workflow to copy and push source to the other repo

[markphelps]

This is a update the publish step for PHP to instead of just creating a tag to:

• create a clone of the repo at the tag
• create a subtree of the flipt-php folder as a new branch
• push just that branch (so only containing the flipt-php folder) up to the already existing https://github.com/flipt-io/flipt-php repo (overwriting main)
• create a new tag for composer

This effectively lets us have a mirror on the flipt-php repo of the code in the flipt-php folder here.