[CI] Added Dependabot for GitHub Actions

Our CI uses external GitHub Actions for different things such as
checking out the repo. This updates Dependabot to check for updates to
these actions and raise a PR whenever there is an update.

Author: AlexandreSinger

.github/dependabot.yml

@@ -9,4 +9,12 @@ updates:
         interval: "monthly"
 
     # Update all submodules
-    directory: /
+    directory: /
+
+  # This will create a PR if there are any updates to the actions used in the CI.
+  - package-ecosystem: "github-actions"
+    # Workflow files stored in the default location of `.github/workflows`
+    directory: "/"
+    # Check for updates once a week
+    schedule:
+      interval: "weekly"