CI: try to fix the tests in the macOS runner

Author: mickeyl

.github/workflows/swift.yml

@@ -15,8 +15,11 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
       - uses: swift-actions/setup-swift@v2
+        if: matrix.os == 'ubuntu-latest'
         with:
           swift-version: ${{ matrix.swift }}
+      - name: Show Swift version
+        run: swift --version
       - uses: actions/checkout@v3
       - name: Build
         run: swift build