Use wasi-sdk 20 pre-release for tests with threads (#2021)

`wasi-sdk-20` pre-release can be used to avoid building `wasi-libc` to enable threads.
It's not possible to use `wasi-sdk-20` pre-release on Ubuntu 20.04 because of
incompatibility with the glibc version:
```bash
/opt/wasi-sdk/bin/clang: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.34' not found
(required by /opt/wasi-sdk/bin/clang)
```

core/iwasm/libraries/lib-wasi-threads/test/build.sh

@@ -5,8 +5,8 @@
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 #
 
+set -eo pipefail
 CC=${CC:=/opt/wasi-sdk/bin/clang}
-WASI_SYSROOT=${WASI_SYSROOT:=~/dev/wasi-libc/sysroot}
 WAMR_DIR=../../../../..
 
 for test_c in *.c; do
@@ -14,7 +14,7 @@ for test_c in *.c; do
 
     echo "Compiling $test_c to $test_wasm"
     $CC \
-        --sysroot $WASI_SYSROOT \
+        $WASI_SYSROOT_OPTION \
         -target wasm32-wasi-threads \
         -pthread -ftls-model=local-exec \
         -z stack-size=32768 \