build: disable vala-pack plugin until upstream fixes vala

We want to keep Nightly builds working, even if that means vala is
disabled. Vala plugin is broken due to bug 779038.

org.gnome.Builder.json

@@ -309,7 +309,7 @@
         },
         {
             "name": "gnome-builder",
-            "config-opts": [ "--enable-tracing", "--enable-debug", "--with-channel=flatpak-nightly" ],
+            "config-opts": [ "--enable-tracing", "--enable-debug", "--with-channel=flatpak-nightly", "--disable-vala-pack-plugin" ],
             "sources": [
                 {
                     "type": "git",