Add `helpers:pinGitHubActionDigests` to renovate.json

The preset will pin third-party GitHub Actions to a full-length commit SHA. This is recommended by Renovate at https://docs.renovatebot.com/upgrade-best-practices/#extends-helperspingithubactiondigests.

Signed-off-by: Suguru Hirahara <did🔑z6MkvVZk1A3KBApWJXv2Ju4H14ErDfRGxh8zxdXSZ4vACDg5>

.github/renovate.json

@@ -1,7 +1,8 @@
 {
 	"$schema": "https://docs.renovatebot.com/renovate-schema.json",
 	"extends": [
-		"config:recommended"
+		"config:recommended",
+		"helpers:pinGitHubActionDigests"
 	],
 	"labels": [
 		"dependencies"