[GitHub Actions] Update SwiftPM workflow

- Add `swift -version`

.github/workflows/swiftpm.yml

@@ -25,8 +25,9 @@ jobs:
     runs-on: macOS-10.14
     steps:
       - uses: actions/checkout@v1
-      - env:
-          DEVELOPER_DIR: /Applications/Xcode_${{ matrix.xcode }}.app
+      - env: { 'DEVELOPER_DIR': '/Applications/Xcode_${{ matrix.xcode }}.app' }
+        run: swift -version
+      - env: { 'DEVELOPER_DIR': '/Applications/Xcode_${{ matrix.xcode }}.app' }
         run: swift test
 
   Linux: