do not use system python3 for serving wasm files

the wasm mimetype is not present 3.6 3.7 so instantiate streaming could fail silently on chrome and firefox the host build is expected to be the latest Python available so just use it

Author: pmp-p

run-python-browser.sh

@@ -4,5 +4,5 @@ cp cpython/builddir/emscripten-browser/python.* browser-ui/worker/
 
 pushd .
 cd browser-ui
-python3 server.py $@
+../cpython/builddir/build/python server.py $@
 popd