`fwupd-refresh.service` uses `DynamicUser=true` which causes systemd
to make `/var/cache/fwupd` a symlink to `/var/cache/private/fwupd`.
Individual units aren't allowed to access this directory, only the ones
with the directive. This means that `fwupd.service` stops working as
soon as a user tries to start `fwupd-refresh.service`.
The bug details are present in
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=941360