pin version of vsc-install in CI

Author: lexming

.github/workflows/unittest.yml

@@ -49,7 +49,7 @@ jobs:
         run: python -m pip install pytest
 
       - name: Install vsc-install
-        run: python -m pip install vsc-install
+        run: python -m pip install vsc-install==0.21.4
 
       - name: Install build-tools
         run: python -m pip install .