ci: Don't build BlueZ backend

We figured out in libfolks that we were accidentally not properly
requiring the `python3-dbusmock` module that is necessary for the BlueZ
tests. We might want to be smarter about this in the future, but let's
just lose the offending backend for now.

Fixes https://gitlab.gnome.org/GNOME/geary/issues/695

org.gnome.Geary.json

@@ -149,6 +149,7 @@
             "buildsystem": "meson",
             "config-opts": [
                 "-Dtelepathy_backend=false",
+                "-Dbluez_backend=false",
                 "-Dinspect_tool=false",
                 "-Dimport_tool=false"
             ],