ci: Use APT_LLVM_V in msan task

Also, use update-alternatives to avoid having to manually specify
clang-${APT_LLVM_V} or llvm-symbolizer-${APT_LLVM_V} everywhere.

ci/test/00_setup_env_native_tsan.sh

@@ -10,7 +10,7 @@ export CONTAINER_NAME=ci_native_tsan
 export CI_IMAGE_NAME_TAG="mirror.gcr.io/ubuntu:24.04"
 export APT_LLVM_V="20"
 export PACKAGES="clang-${APT_LLVM_V} llvm-${APT_LLVM_V} libclang-rt-${APT_LLVM_V}-dev libc++abi-${APT_LLVM_V}-dev libc++-${APT_LLVM_V}-dev python3-zmq"
-export DEP_OPTS="CC=clang-${APT_LLVM_V} CXX='clang++-${APT_LLVM_V} -stdlib=libc++'"
+export DEP_OPTS="CC=clang CXX='clang++ -stdlib=libc++'"
 export GOAL="install"
 export BITCOIN_CONFIG="-DWITH_ZMQ=ON -DSANITIZERS=thread \
 -DAPPEND_CPPFLAGS='-DARENA_DEBUG -DDEBUG_LOCKORDER -DDEBUG_LOCKCONTENTION -D_LIBCPP_REMOVE_TRANSITIVE_INCLUDES'"