[wasm] Set frozen cache emscripten variable conditionally (dotnet#95015)

* Do not set frozen cache when user overrides emscripten cache

We should not set FROZEN_CACHE variable when using different cache
than the default one from our nuget.

That should also help with other issues related to emscripten cache
and act as a workaround.

* Feedback

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

@@ -286,7 +286,7 @@
       <EmscriptenEnvVars Include="PYTHONPATH=$(EmscriptenPythonToolsPath)" Condition="'$(OS)' == 'Windows_NT'" />
       <EmscriptenEnvVars Include="PYTHONHOME=" Condition="'$(OS)' == 'Windows_NT'" />
       <EmscriptenEnvVars Include="EM_CACHE=$(WasmCachePath)" Condition="'$(WasmCachePath)' != ''" />
-      <EmscriptenEnvVars Include="EM_FROZEN_CACHE=True" />
+      <EmscriptenEnvVars Include="EM_FROZEN_CACHE=True" Condition="'$(WasmCachePath)' == '$(EmscriptenCacheSdkCacheDir)'" />
       <EmscriptenEnvVars Include="DISABLE_LEGACY_JS_INTEROP=1" Condition="'$(WasmEnableLegacyJsInterop)' == 'false'" />
       <EmscriptenEnvVars Include="DISABLE_LEGACY_JS_INTEROP=0" Condition="'$(WasmEnableLegacyJsInterop)' != 'false'" />
       <EmscriptenEnvVars Include="WASM_ENABLE_SIMD=1" Condition="'$(WasmEnableSIMD)' != 'false'" />