bug#47260: Wrapping binaries in MediaGoblin Guix Package

[jgart]

Just a note that we'll probably have to wrap the binaries that are in this 
module when we finish making the guix package:

mediagoblin/media_types/pdf/processing.py

I see that pdftocairo, pdfinfo, and unoconv probably need to be wrapped to point
to the executable that is in /gnu/store.

See this issue for a similar discussion about wrapping binaries with guix:

https://issues.guix.gnu.org/50833