ci: build V using V

.github/workflows/ci.yml

@@ -13,8 +13,10 @@ jobs:
       run: |
         brew install freetype glfw openssl
         export LIBRARY_PATH="$LIBRARY_PATH:/usr/local/opt/openssl/lib/"
-    - name: Build v
+    - name: Build V
       run:  make && ./v -o v compiler
+    - name: Build V using V
+      run:  ./v -o v2 compiler && ./v2 -o v3 compiler
     - name: Test v->c
       run: ./v test v
     - name: Test v->js