ci: Disable a11y bus for tests

While the regular session bus is provided by `dbus-run-session`, the
a11y bus is spawn by the "normal" D-Bus daemon (that is, dbus-broker
in F30). This currently fails, either due to a bug or some missing
dependencies in the container environment. But as we don't actually
need the additional bus, just disable it via the environment to make
not break tests when updating the base image to F30.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/582

.gitlab-ci.yml

@@ -36,6 +36,7 @@ test-mutter:
     XDG_RUNTIME_DIR: "$CI_PROJECT_DIR/runtime-dir"
     GSETTINGS_SCHEMA_DIR: "$CI_PROJECT_DIR/build/data"
     MALLOC_CHECK_: "3"
+    NO_AT_BRIDGE: "1"
   script:
     - mkdir -m 700 $XDG_RUNTIME_DIR
     - glib-compile-schemas $GSETTINGS_SCHEMA_DIR