[wasm] Unset PYTHONHOME (#67282)

Unset PYTHONHOME environment variable. We don't use emsdk.bat, which
unsets PYTHONHOME and PYTHONPATH variables to not clash with emscripten
python. So we should make sure the PYTHONHOME and PYTHONPATH are not
pointing to other python locations.

Together with previous PYTHONPATH
[change](https://github.com/dotnet/runtime/pull/63894/commits/d9b8aefcbf0efc4692c9ca46b323efcf86bda961)
it should fix https://github.com/dotnet/runtime/issues/65859

src/mono/wasm/build/WasmApp.Native.targets

@@ -256,6 +256,7 @@
 
       <EmscriptenEnvVars Include="EMSDK_PYTHON=$(EmscriptenPythonToolsPath)python.exe" Condition="'$(OS)' == 'Windows_NT'" />
       <EmscriptenEnvVars Include="PYTHONPATH=$(EmscriptenPythonToolsPath)" Condition="'$(OS)' == 'Windows_NT'" />
+      <EmscriptenEnvVars Include="PYTHONHOME=" Condition="'$(OS)' == 'Windows_NT'" />
       <EmscriptenEnvVars Include="EM_CACHE=$(WasmCachePath)" Condition="'$(WasmCachePath)' != ''" />
     </ItemGroup>