ci: test cross compilation to linux on macos

.github/workflows/ci.yml

@@ -149,6 +149,8 @@ jobs:
 #      run: ./v -freestanding -o bare vlib/os/bare/bare_example_linux.v
     - name: v self compilation
       run: ./v -o v2 cmd/v && ./v2 -o v3 cmd/v && ./v3 -o v4 cmd/v
+    - name: cross compilation to Linux
+      run: ./v -os linux cmd/v
     - name: Fixed tests
       run: |
         ./v test-fixed