Enable dependabot scan on github actions (#4704)

Since we are using GitHub Actions in CoreDNS repo and we are already
using Dependabot for security/version scans on golang code,
it makes sense to enable security/version scans on GitHub Actions as well.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

.github/dependabot.yml

@@ -5,3 +5,8 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
+
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"