dependabot: run it also for staging crates

We forgot to tell dependabot to also run in the "staging" nested
workspace. Let's enable it.

Closes #536

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>

.github/dependabot.yml

@@ -11,6 +11,17 @@ updates:
       vhost-device:
         patterns:
           - "*"
+- package-ecosystem: cargo
+  directory: "/staging/"
+  schedule:
+    interval: weekly
+  allow:
+  - dependency-type: direct
+  - dependency-type: indirect
+  groups:
+      vhost-device:
+        patterns:
+          - "*"
 - package-ecosystem: gitsubmodule
   directory: "/"
   schedule: