fix(dotnet): fix Windows CI unused-import/dead-code in dotnet_double_dash_test.rs

The test itself is #[cfg(unix)] (it spawns a /bin/sh stub using unix file
permissions), but its Command import and shell_quote helper weren't gated
the same way -- on Windows the test compiled out entirely, leaving both
genuinely unused and tripping -D unused-imports / -D dead-code.

Nest both inside the test function's body instead of gating them separately:
since the whole function is already #[cfg(unix)], that one attribute now
covers everything, and Linux/macOS behavior is unaffected (cargo test --all
still passes the same 2765 tests).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>