Put pip bin directory at front of $PATH in pip_install.sh

build_tools/pip_install.sh

 #!/bin/bash
 # Copyright 2014 The Native Client Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Installs required python modules using 'pip'.
 # Always use a locally installed version of pip rather than relying
 # on the system version since the behaviour and command line flags
 # for pip inself vary between versions.
 
 SCRIPT_DIR="$(cd $(dirname $0) && pwd)"
 ARGS="--user --no-compile"
 
 cd "${SCRIPT_DIR}/.."
 
 export PYTHONUSERBASE=$PWD/out/pip
 pip_bin_dir=$PYTHONUSERBASE/bin
 pip_bin=$pip_bin_dir/pip
-export PATH=$PATH:$pip_bin_dir
+export PATH=$pip_bin_dir:$PATH
 
 if [ ! -f "$pip_bin" ]; then
   # On first run install pip directly from the network
   echo "Installing pip.."
   # Use local file rather than pipeline so we can detect failure of the curl
   # command.
   curl --silent --show-error https://bootstrap.pypa.io/get-pip.py > get-pip.py
   python get-pip.py --force-reinstall --user
   rm -f get-pip.py
   hash -r
 fi
 
 set -x
 # Pin locally install pip to a specific version
 pip install ${ARGS} "pip==6.0.6"
 
 # At this point we know we have good pip install in $PATH and we can use
 # it to install the requirements.
 pip install ${ARGS} -r requirements.txt