ci: configure Dependabot to update GitHub Actions

.github/dependabot.yml

@@ -7,3 +7,10 @@ updates:
     commit-message:
       prefix: "chore(cargo)"
     open-pull-requests-limit: 50
+
+  # Keep GitHub Actions up to date.
+  # <https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot>
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"