ci: Check github-actions deps 3x in year

GitHub Actions are updated frequently, but are important mostly during
_opinions_ releasing (which is rare). Go modlues are updated rarely. Lower
check frequency means less time spent on merges.

Author: macie

.github/dependabot.yml

@@ -3,7 +3,8 @@ updates:
   - package-ecosystem: github-actions
     directory: /
     schedule:
-      interval: monthly
+      interval: "cron"
+      cronjob: "5 4 2 */4 *"
 
   - package-ecosystem: gomod
     directory: /