Avoid unused thread_id warning and recompile multi-module sample (#3033)

- Unused variable warning (when building in release mode):
```bash
wasm-micro-runtime/samples/wasi-threads/wasm-apps/no_pthread.c:44:9:
warning: variable 'thread_id' set but not used [-Wunused-but-set-variable]
    int thread_id;
```
- When using the multi-module example and changing the files in `wasm-apps`,
  they don't get recompiled when the sample is rebuilt. Adding `BUILD_ALWAYS`
  as it's done in other samples.

samples/multi-module/CMakeLists.txt

@@ -132,6 +132,7 @@ endif()
 # .c -> .wasm
 ExternalProject_Add(WASM_MODULE
   SOURCE_DIR         ${CMAKE_CURRENT_SOURCE_DIR}/wasm-apps
+  BUILD_ALWAYS TRUE
   UPDATE_COMMAND     ""
   PATCH_COMMAND      ""
   CONFIGURE_COMMAND  ${CMAKE_COMMAND}