Allow setting LLVM_ROOT via an env var (#9777)
Allow setting LLVM_ROOT via an env var (#9777)

This is a follow-up on #9535. 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
```

Signed-off-by: Raul Gutierrez Segales <f25f6873bbbde69f1fe653b3e6bd40d543b8d0e0@pinterest.com>
eklitzke/envoy,envoyproxy/envoy,lyft/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,lizan/envoy,envoyproxy/envoy-wasm,istio/envoy,lizan/envoy,envoyproxy/envoy,eklitzke/envoy,lyft/envoy,lyft/envoy,eklitzke/envoy,eklitzke/envoy,lyft/envoy,lyft/envoy,istio/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,lizan/envoy,envoyproxy/envoy,lizan/envoy,istio/envoy,envoyproxy/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy-wasm,envoyproxy/envoy,envoyproxy/envoy-wasm,eklitzke/envoy,envoyproxy/envoy,eklitzke/envoy,envoyproxy/envoy,envoyproxy/envoy,istio/envoy,istio/envoy,lizan/envoy,istio/envoy,envoyproxy/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,lizan/envoy,lyft/envoy,istio/envoy