Use Clang when building JAX via build/build.py

Previously the CC=clang CXX=clang++ variables had no effect and we were
building using GCC. Now that build/build.py has a supported `use_clang`
flag, use that instead.

Author: andportnoy

Diff for: .github/container/build-jax.sh

@@ -277,7 +277,8 @@ pushd $SRC_PATH_JAX
 # Delete old wheel if one already exist.
 rm -rf dist/j*.whl
 
-time CC=clang CXX=clang++ python build/build.py \
+time python build/build.py \
+    --use_clang \
     --enable_cuda \
     --cuda_path=$TF_CUDA_PATHS \
     --cudnn_path=$TF_CUDNN_PATHS \