tests: Use nonet when validating appstream data

Othewise we fail when building without network (e.g. when building
the Debian package with sbuild/pbuilder)
3 jobs for nonet in 4 minutes and 5 seconds
Status Name Job ID Coverage
  Build
passed build-debian-gcc #5000
librem5

00:01:22

 
  Test
passed test:debian-gcc #5001
librem5

00:01:17

 
  Package
passed build-debian-package #5002
librem5

00:01:24