libhtml5.a has absolute header path

Step to reproduce:
* Use emscripten 4.0.0
* `embuilder build libhtml5`
* `ar x libhtml5.a`
* `cat emscripten_wget.o`
* See something like below, where `/mnt/storage/github/emsdk` is the place I clone emsdk.

```
.debug_strclang version 20.0.0git (https:/github.com/llvm/llvm-project 322eb1a92e6d4266184060346616fa0dbe39e731)/emsdk/emscripten/system/lib/html5/emscripten_wget.c/emsdk/emscriptenemscripten_wget_datacharintopenwritelongssize_tunsigned longsize_tclosefreestrdupmkdirunsigned intmode_t__errno_locationemscripten_wgetmkdirsbuffernumerrorfileurlfdcopycresult����
system/lib/html5cache/sysroot/include/emscriptencache/sysroot/includecache/sysroot/include/bits/mnt/storage/github/emsdk/upstream/lib/clang/20/includecache/sysroot/include/sysemscripten_wget.cwget.hfcntl.hunistd.halltypes.h__stddef_size_t.hstdlib.hstring.hstat.herrno.h
```