fix: change staging client secret to retrieve from secrets

.github/workflows/pr.yml

@@ -79,7 +79,7 @@ jobs:
             -H "Content-Type: application/x-www-form-urlencoded" \
             -d "grant_type=client_credentials" \
             -d "client_id=${{ vars.STAGING_CLIENT_ID }}" \
-            -d "client_secret=${{ vars.STAGING_CLIENT_SECRET }}"
+            -d "client_secret=${{ secrets.STAGING_CLIENT_SECRET }}"
           )
 
           access_token=$(echo "$response" | jq -r '.access_token')