Fix Windows MSVC unit tests build by setting up developer environment

The `cl` compiler was not in PATH because the Visual Studio developer
environment needs to be set up before calling MSVC tools directly.
Added `ilammy/msvc-dev-cmd@v1` action to configure the environment.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -148,6 +148,9 @@ jobs:
         run: |
           .\build\Release\loader_example.exe models\Cube\Cube.gltf
 
+      - name: Setup MSVC environment
+        uses: ilammy/msvc-dev-cmd@v1
+
       - name: Build unit tests
         run: |
           cd tests