Use wasi-sdk-20 to build wasi-threads cases in CI (#2095)

wasi-sdk-20 supports older versions of glibc and allow us to use it in the CI
with Ubuntu 20.04.

Refer to
  https://github.com/WebAssembly/wasi-sdk/releases/tag/wasi-sdk-20
And #2021 for previous upgrade to wasi-sdk-20 pre-release.

core/iwasm/libraries/lib-socket/test/build.sh

@@ -11,7 +11,6 @@ for file in "${files[@]}"
 do
     echo $file
     $CC \
-        $WASI_SYSROOT_OPTION \
         --target=wasm32-wasi-threads \
         -I../inc \
         ../src/wasi/wasi_socket_ext.c -pthread -ftls-model=local-exec \