fix: check cargo-xwin availability using subcommand instead of binary name

Change cargo-xwin detection from `command -v cargo-xwin` to `cargo xwin --version` to properly verify cargo subcommand availability on Linux systems cross-compiling to Windows targets.

desktop-client/scripts/build-tunnel-helper.sh

@@ -32,7 +32,7 @@ if [ "${TARGET}" = "i686-pc-windows-msvc" ]; then
     MINGW64_NT*|MSYS_NT*|CYGWIN_NT*)
       ;;
     Linux)
-      if ! command -v cargo-xwin >/dev/null 2>&1; then
+      if ! cargo xwin --version >/dev/null 2>&1; then
         echo "Linux cross-builds for Windows require cargo-xwin."
         echo "Install it with:"
         echo "  cargo install --locked cargo-xwin"