Validate appdata file with nonet flag

By default, appstream-util checks if URLs listed in the appdata file are
resolvable.  Tuba's appdata file includes screenshot URLs.  Most Linux
distro build environments do not allow outbound network connections.
Without a network connection, the validation fails.  Fedora recommends
using the `--nonet` flag to skip that part.

https://docs.fedoraproject.org/en-US/packaging-guidelines/AppData/#_app_data_validate_usage

data/meson.build

@@ -55,6 +55,6 @@ if appstream_util.found()
     test(
         'Validate appstream file',
         appstream_util,
-        args: ['validate-relax', appstream_file],
+        args: ['validate-relax', '--nonet', appstream_file],
     )
 endif