ci(pull-requests): report total attempt count in prepare-env retry log

Address review feedback from coderabbitai on
.github/workflows/pull-requests.yaml:235: $attempt is incremented only on
failure, so first-try success printed "completed after 0 attempts". Use
$((attempt + 1)) so the message reflects the total number of runs.

Signed-off-by: Myasnikov Daniil <myasnikovdaniil2001@gmail.com>

.github/workflows/pull-requests.yaml

@@ -232,7 +232,7 @@ jobs:
             fi
             echo "Attempt $attempt failed, retrying..."
           done
-          echo "Prepare environment completed after $attempt attempts"
+          echo "Prepare environment completed after $((attempt + 1)) attempts"
 
       # ▸ Install Cozystack
       - name: Install Cozystack into sandbox