Pin wasi-sdk for C#

Author: alexcrichton

.github/workflows/main.yml

@@ -103,6 +103,10 @@ jobs:
       with:
         dotnet-version: '9.x'
       if: matrix.lang == 'csharp'
+    - uses: ./.github/actions/install-wasi-sdk
+      if: matrix.lang == 'csharp'
+      with:
+        version: 29
 
     # As of this writing async tests require [a patched build of
     # Go](https://github.com/dicej/go/releases/tag/go1.25.5-wasi-on-idle).