[github-actions] avoid dependabot pull requests in forks

OpenThread forks typically inherit all commits in the repo, including
the `dependabot` changes, so it's not needed to generate the same PRs
in the forks as well.

.github/dependabot.yml

@@ -29,6 +29,7 @@
 version: 2
 updates:
   - package-ecosystem: "github-actions"
+    if: github.repository == "openthread/openthread"
     directory: "/"
     schedule:
       interval: "weekly"