Fix cross-compilation toolchain installation for all platforms

- macOS: Use messense/macos-cross-toolchains tap for musl toolchain
- Windows: Fix PowerShell syntax for creating cargo config
- Add verification step to check musl target installation
- Update builder to try platform-specific linker names
- Fix YAML syntax error in Windows config generation

🤖 Generated with [Claude Code](https://claude.ai/code)

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

example/hello-krust/src/main.rs

@@ -1,4 +1,4 @@
 fn main() {
     println!("Hello from krust example!");
     println!("This is running inside a container built with krust.");
-}
+}