Add note that system packages are not supported

Author: tiran

Tools/wasm/wasm_build.py

@@ -9,7 +9,8 @@
 "emscripten-browser", and "wasi".
 
 Emscripten builds require a recent Emscripten SDK. The tools looks for an
-activated EMSDK environment (". /path/to/emsdk_env.sh").
+activated EMSDK environment (". /path/to/emsdk_env.sh"). System packages
+(Debian, Homebrew) are not supported.
 
 WASI builds require WASI SDK and wasmtime. The tool looks for 'WASI_SDK_PATH'
 and falls back to /opt/wasi-sdk.