Add support for the fonts binded in pressure-vessel
Pressure-vessel can bind the host fonts, when it creates a new container. In order for it to work though, we need to tell fontconfig where these fonts are located.
Related MR steam-runtime-tools!175 (merged)
Both MRs are needed for this functionality to work but it's not a problem if they land at different times.
The same patch should be applied to Soldier too.
/cc @smcv