add GitHub Actions dependency updates

Author: s5suzuki

.github/dependabot.yml

@@ -7,3 +7,14 @@ updates:
     ignore:
       - dependency-name: "*"
         update-types: ["version-update:semver-patch"]
+
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    commit-message:
+      prefix: ""
+    groups:
+      github-actions-dependencies:
+        patterns:
+          - "*"