infra: Use dependabot to automatically update GitHub actions.

Signed-off-by: Tim 'mithro' Ansell <me@mith.ro>

.github/workflows/dependabot-gha.yml

@@ -0,0 +1,9 @@
+# Set update schedule for GitHub Actions
+version: 2
+updates:
+
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      # Check for updates to GitHub Actions every weekday
+      interval: "daily"