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-wasi-threads/test/build.sh

@@ -14,7 +14,6 @@ for test_c in *.c; do
 
     echo "Compiling $test_c to $test_wasm"
     $CC \
-        $WASI_SYSROOT_OPTION \
         -target wasm32-wasi-threads \
         -pthread -ftls-model=local-exec \
         -z stack-size=32768 \