ci: fix Windows Swift setup - use swift-build parameter

- Change from 'swift-version' to 'swift-build: 6.1-RELEASE'
- Required by latest compnerd/gha-setup-swift action
- Resolves: 'swift-build is required when using swift.org source' error

Author: coenttb

.github/workflows/ci.yml

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