Fix fonts errors

[mrakitin]

Azure jobs (e.g. here) throw these errors:

Unable to revert mtime: /usr/share/fonts
Unable to revert mtime: /usr/share/fonts/cmap
Unable to revert mtime: /usr/share/fonts/truetype
Unable to revert mtime: /usr/share/fonts/type1
Unable to revert mtime: /usr/share/fonts/cmap/adobe-cns1
Unable to revert mtime: /usr/share/fonts/cmap/adobe-gb1
Unable to revert mtime: /usr/share/fonts/cmap/adobe-japan1
Unable to revert mtime: /usr/share/fonts/cmap/adobe-japan2
Unable to revert mtime: /usr/share/fonts/cmap/adobe-korea1
Unable to revert mtime: /usr/share/fonts/truetype/dejavu
Unable to revert mtime: /usr/share/fonts/truetype/liberation
Unable to revert mtime: /usr/share/fonts/type1/gsfonts

https://github.com/bluesky/tutorial/blob/master/binder/postBuild#L3 can be used as an example.

@tacaswell:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=909728
because mpl builds the font cache on first import
doing it there means it won't happen later when things might timeout etc