Qpdf Installer for GitHub Actions

This GitHub action installs qpdf:

- uses: yegor256/qpdf-action@master

I'm sorry, but it works on Ubuntu only. If you know how to make it work no MacOS and Windows, please, submit a pull request, I will gladly accept it.

Also, there is no caching now. This means that every time it will take about a few minutes to install qpdf, on every single build. This is terrible, but I don't know how to fix this. If you know, please submit a pull request.