message: Allow setting LLVM_ROOT via an env var
  This is a follow-up on With this change, I can
  run `ci/do_ci.sh` without docker on OS X. E.g.:
  ```
  NUM_CPUS=2 \
  ENVOY_BUILD_TARGET="//pinterest:envoy" \
  BUILD_DIR=$(pwd)/build \
  ENVOY_SRCDIR=$(pwd)/envoy/ \
  bash -x envoy/ci/do_ci.sh bazel.dev
  ```
diff: @@ -80,7 +80,7 @@ function cleanup() {
  cleanup
  trap cleanup EXIT
  -export LLVM_ROOT=/opt/llvm
  +export LLVM_ROOT="${LLVM_ROOT:-/opt/llvm}"
  "$(dirname "$0")"/../bazel/setup_clang.sh "${LLVM_ROOT}"
  [[ "${BUILD_REASON}" != "PullRequest" ]] && BAZEL_EXTRA_TEST_OPTIONS+=" --nocache_test_results --test_output=all"
  