[ci] Get REGISTRY from vars, not secrets

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

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

Signed-off-by: Timofei Larkin <lllamnyp@gmail.com>

.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]