ci: Drop BASE_SCRATCH_DIR from LIBCXX_DIR

Using a hard-coded path avoids non-determinism issues and improves CI
UX.

ci/test/00_setup_env_native_msan.sh

@@ -7,7 +7,7 @@
 export LC_ALL=C.UTF-8
 
 export CI_IMAGE_NAME_TAG="ubuntu:22.04"
-LIBCXX_DIR="${BASE_SCRATCH_DIR}/msan/cxx_build/"
+LIBCXX_DIR="/msan/cxx_build/"
 export MSAN_FLAGS="-fsanitize=memory -fsanitize-memory-track-origins=2 -fno-omit-frame-pointer -g -O1 -fno-optimize-sibling-calls"
 LIBCXX_FLAGS="-nostdinc++ -nostdlib++ -isystem ${LIBCXX_DIR}include/c++/v1 -L${LIBCXX_DIR}lib -Wl,-rpath,${LIBCXX_DIR}lib -lc++ -lc++abi -lpthread -Wno-unused-command-line-argument"
 export MSAN_AND_LIBCXX_FLAGS="${MSAN_FLAGS} ${LIBCXX_FLAGS}"