ci: add swift-version parameter for Windows Swift setup

- Add swift-version: "6.1" alongside swift-build parameter
- Both parameters required by compnerd/gha-setup-swift@main with swift.org source
- Fixes "swift-version is required when using swift.org source" error

Author: coenttb

.github/workflows/ci.yml

@@ -65,6 +65,7 @@ jobs:
 
       - uses: compnerd/gha-setup-swift@main
         with:
+          swift-version: "6.1"
           swift-build: "6.1-RELEASE"
 
       - name: Build