[chore] enable github action pin digest helper in renovatebot (#12799)

#### Description

As part of the security slam, we've been working to pin digests in
GitHub actions. There's some tools that can run through and pin
everything, but without the notes and comments of every versioned, it
can be difficult to understand what is actually being pinned. Thus, I'm
opting to add the [renovate
helper](https://docs.renovatebot.com/modules/manager/github-actions/#digest-pinning-and-updating)
for this in GitHub actions which should automatically manage version
pinning.

#### Link to tracking issue
Related to
[#86](open-telemetry/sig-security#87 (comment))

#### Testing

Ran `npx --yes --package renovate -- renovate-config-validator
renovate.json` after updating the config.

Author: adrielp

renovate.json

@@ -8,7 +8,8 @@
     "go": "1.23"
   },
   "extends": [
-    "config:recommended"
+    "config:recommended",
+    "helpers:pinGitHubActionDigests"
   ],
   "schedule": [
     "on tuesday"