how to make the change of system lib source to work?

**Version of emscripten/emsdk:**
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.64 (a1fe3902bf73a3802eae0357d273d0e37ea79898)
clang version 19.0.0git (https:/github.com/llvm/llvm-project 4d8e42ea6a89c73f90941fd1b6e899912e31dd34)
Target: wasm32-unknown-emscripten
Thread model: posix

i make a little change the emscripten_fetch.c file of the fetch lib,
https://github.com/emscripten-core/emscripten/blob/5a1578a50cf0a4d7120813984c9a71cccc97e61a/system/lib/fetch/emscripten_fetch.c#L88
to something like this:

```
printf("change calloc to malloc\n");
emscripten_fetch_t* fetch = (emscripten_fetch_t*)malloc(sizeof(emscripten_fetch_t));
if (!fetch) {
return NULL;
}
memset(fetch, 0, sizeof(emscripten_fetch_t));
```
after change the code ,i remove the libfetch*.a in cache/sysroot/lib folder, and after compile , in runtime, i see the 'change calloc to malloc' output in the browser console, but when i see the wasm decode in web browser, there is no malloc call, the calloc call is still there, why?