Add Dependabot config for GitHub Actions (#31)

This will help ensure that upstream GitHub actions stay up to date. It
operationalizes updates like
#30 such that
they're never missed.


https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot

### Testing
You can see the PRs that this config will open in my fork here:


https://github.com/wadells/devbox-install-action/pulls?q=+is%3Apr+author%3Aapp%2Fdependabot+

Author: wadells

.github/dependabot.yml

@@ -0,0 +1,7 @@
+version: 2
+updates:
+  - package-ecosystem: github-actions
+    directory: "/"
+    schedule:
+      interval: weekly
+      day: monday