meson: Validate appstream file with "--nonet" arg

We do not need network connection to validate our appstream file.
Make it easier to run tests in containers that do not have network access.

Closes #307

data/meson.build

@@ -3,7 +3,7 @@ iconsdir = join_paths(datadir, 'icons', 'hicolor')
 appstream_util = find_program('appstream-util', required: false)
 if appstream_util.found()
   test('Validate appstream file', appstream_util, args: [
-    'validate-relax',
+    'validate-relax', '--nonet',
     join_paths(meson.current_source_dir(), 'com.github.rafostar.Clapper.metainfo.xml')
   ])
 endif