dependabot: Manage GitHub Action workflows also

Signed-off-by: Stephen Finucane <[email protected]>

Author: stephenfin

.github/dependabot.yml

@@ -9,3 +9,7 @@ updates:
     ignore:
       - dependency-name: "*"
         update-types: ["version-update:semver-patch"]
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "monthly"