[ci] Get REGISTRY from vars, not secrets (#1423)

## What this PR does

This patch sources the REGISTRY env var from GitHub actions variables
instead of secrets, so pull requests from forked repos work correctly.

### Release note

```release-note
[ci] Source the REGISTRY env var from actions' variables, not secrets,
so external pull requests can work.
```

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Chores**
* Updated CI configuration to source the container registry setting from
organization variables instead of secrets, improving maintainability and
visibility of build settings.
* No impact to application features, functionality, or performance;
builds and deployments continue to operate as before.
* No action required from users or admins; this is an internal workflow
refinement.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

.github/workflows/pull-requests.yaml

@@ -1,7 +1,7 @@
 name: Pull Request
 
 env:
-  REGISTRY: ${{ secrets.OCIR_REPO }}
+  REGISTRY: ${{ vars.OCIR_REPO }}
 on:
   pull_request:
     types: [opened, synchronize, reopened]