try to validate linkage on all platforms

The build should also build a binary, which needs to be linked.

.github/workflows/cross-platform-testing.yml

@@ -64,6 +64,8 @@ jobs:
           echo "target flag is: ${{ env.TARGET_FLAGS }}"
           echo "target dir is: ${{ env.TARGET_DIR }}"
 
+      - name: Build (and link)
+        run: ${{ env.CARGO }} build ${{ env.TARGET_FLAGS }}
       - name: Run tests
         run: ${{ env.CARGO }} test ${{ env.TARGET_FLAGS }}
       - name: Run fmt