use the right pip

build.sh

@@ -118,8 +118,9 @@ else
   # The following line installs pyarrow from S3, these wheels have been
   # generated from https://github.com/ray-project/arrow-build from
   # the commit listed in the command.
-  pip install --target=$ROOT_DIR/python/ray/pyarrow_files pyarrow==0.12.0-RAY \
-              --find-links https://s3-us-west-2.amazonaws.com/arrow-wheels/bf0f3a15e0d6583cfaedbca508627b06746fa41a/index.html
+  $PYTHON_EXECUTABLE -m pip install \
+      --target=$ROOT_DIR/python/ray/pyarrow_files pyarrow==0.12.0-RAY \
+      --find-links https://s3-us-west-2.amazonaws.com/arrow-wheels/bf0f3a15e0d6583cfaedbca508627b06746fa41a/index.html
   bazel build //:ray_pkg -c opt
   # Copy files and skip existing files
   cp -r -n $ROOT_DIR/bazel-genfiles/ray_pkg/ray $ROOT_DIR/python || true