use github env vars in doxygen builds about doxygen-action

[mattnotmitt]

Hi @2bndy5, this issue isn't really related to this action, but I've just tried it out in my test repo. If you set up your Doxyfile as you described and then edit your workflow file to set up the variable to be passed to my action. I don't have any tags set up on that repo to test with, but building from master, ${{ github.ref }} gives this output to the PROJECT_NUMBER option and I assume this would just give you the tag name if you built on that event. Hope this helps!