chore: let Dependabot keep GitHub Actions up to date (#138)

Author: erwan-joly

.github/dependabot.yml

@@ -5,3 +5,7 @@ updates:
   schedule:
     interval: daily
   open-pull-requests-limit: 50
+- package-ecosystem: github-actions
+  directory: "/"
+  schedule:
+    interval: weekly