github: Upgrade checkout@v4 to checkout@v6

checkout@v5 should be the same as checkout@v4, just running on
Node 24. checkout@v6 has got a few other minor changes that
shouldn't make any difference; opt in to newer behaviours now.

.github/workflows/test-llvm.yml

@@ -64,7 +64,7 @@ jobs:
           echo "c:\llvm-mingw\bin" | Out-File -FilePath $Env:GITHUB_PATH -Encoding utf8 -Append
           echo "c:\llvm-mingw\python\bin" | Out-File -FilePath $Env:GITHUB_PATH -Encoding utf8 -Append
           echo "PYTHON_EXE=c:/llvm-mingw/python/bin/python3.exe" | Out-File -FilePath $Env:GITHUB_ENV -Encoding utf8 -Append
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Checkout llvm-project
         env:
           LLVM_VERSION: ${{needs.prepare.outputs.LLVM_VERSION}}