Guix,

Just leaving this here so I don't forget.

Sometimes fonts break, and after a few frustrating, impotent runs 
of ‘fc-cache -rv’ you notice:

  /var/cache/fontconfig: not cleaning unwritable cache directory

Deleting this stale (root-owned) directory makes fonts fun again. 
Happens about once or twice a month here.

Kind regards,

T G-R