fix(lint): add appstream-screenshots-not-mirrored-in-ostree exception for virtualbox

Same repo-lint check that thunderbird-nightly needed. builddir and repo
lint are separate steps with different error codes.

Assisted-by: Claude Sonnet 4.6 via OpenCode

Author: castrojo

flatpaks/virtualbox/exceptions.json

@@ -1,6 +1,7 @@
 {
     "org.virtualbox.VirtualBox": [
         "appid-filename-mismatch",
-        "appstream-external-screenshot-url"
+        "appstream-external-screenshot-url",
+        "appstream-screenshots-not-mirrored-in-ostree"
     ]
 }