Change checkout ref for Alive2 in build workflow

Updated the checkout reference for the Alive2 repository.

Author: dtcxzyw

.github/workflows/build.yml

@@ -37,7 +37,6 @@ jobs:
       uses: actions/checkout@v4
       with:
         repository: AliveToolkit/alive2
-        ref: '20a8472b77ba8a1f0b172059d422a55cb4dff120'
         path: alive2
 
     - name: Compile Alive2