Fixes a regression introduce in 01334a61c7
that would prevent system cross-compilation (with "guix system image
--target=...") due to 'font-gnu-unifont' depending on 'perl-gd', which
currently cannot be cross-compiled.
* gnu/services/base.scm (%default-console-font, kmscon-service-type):
Refer to 'font-gnu-unifont' with 'ungexp-native'.