[sanitizer] Fix libcxx cache existence check in symbolizer build (#149249)

The script incorrectly checked ${LLVM_BUILD}/build.ninja to determine if
cached libcxx is available, while it should be checking the actual
libcxx build directory at ${LIBCXX_BUILD}/build.ninja.

compiler-rt/lib/sanitizer_common/symbolizer/scripts/build_symbolizer.sh

@@ -87,7 +87,7 @@ AR="${AR}" CC="${CC}" CFLAGS="$FLAGS -Wno-deprecated-non-prototype" RANLIB=/bin/
 make -j libz.a
 
 # Build and install libcxxabi and libcxx.
-if [[ ! -f ${LLVM_BUILD}/build.ninja ]]; then
+if [[ ! -f ${LIBCXX_BUILD}/build.ninja ]]; then
   rm -rf "${LIBCXX_BUILD}" "${LIBCXX_INSTALL}"
   mkdir -p ${LIBCXX_BUILD} ${LIBCXX_INSTALL}
   cd ${LIBCXX_BUILD}