CI: expose build PDF as an artifact

@mmuetzel suggested:

> If you'd like to start from an example, Octave has the following in its CI which you could probably adapt:
> 
> https://github.com/gnu-octave/octave/blob/22acaba9b76a52c1ddcfca393932a244728fc18c/.github/workflows/make-mingw.yaml#L312