[libcxx] CI: set symbolizer for bootstrapping build

Setting the symbolizer is required for getting a pretty
stack trace when Clang crashes.

Signed-off-by: Matheus Izvekov <mizvekov@gmail.com>

Differential Revision: https://reviews.llvm.org/D132807

libcxx/utils/ci/buildkite-pipeline.yml

@@ -372,6 +372,7 @@ steps:
     env:
         CC: "clang-${LLVM_HEAD_VERSION}"
         CXX: "clang++-${LLVM_HEAD_VERSION}"
+        LLVM_SYMBOLIZER_PATH: "/usr/bin/llvm-symbolizer-${LLVM_HEAD_VERSION}"
     agents:
       queue: "libcxx-builders"
       os: "linux"