Temporarily use ruby_4_0 branch for cross-compile testing

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hsbt

.github/workflows/crosscompile.yml

@@ -45,6 +45,7 @@ jobs:
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
+          ref: ruby_4_0
           sparse-checkout-cone-mode: false
           sparse-checkout: /.github
           persist-credentials: false