Fix MacOS Python 3.5 gevent build

tools/run_tests/helper_scripts/build_python.sh

@@ -165,24 +165,33 @@ pip_install_dir() {
   cd "$PWD"
 }
 
+# On library/version/platforms combos that do not have a binary
+# published, we may end up building a dependency from source. In that
+# case, several of our build environment variables may disrupt the
+# third-party build process. This function pipes through only the
+# minimal environment necessary.
+pip_install() {
+  /usr/bin/env -i PATH="$PATH" $VENV_PYTHON -m pip install $@
+}
+
 case "$VENV" in
   *py36_gevent*)
   # TODO(https://github.com/grpc/grpc/issues/15411) unpin this
-  $VENV_PYTHON -m pip install gevent==1.3.b1
+  pip_install gevent==1.3.b1
   ;;
   *gevent*)
-  $VENV_PYTHON -m pip install -U gevent
+  pip_install -U gevent
   ;;
 esac
 
-$VENV_PYTHON -m pip install --upgrade pip==19.3.1
-$VENV_PYTHON -m pip install --upgrade setuptools
-$VENV_PYTHON -m pip install --upgrade cython
-$VENV_PYTHON -m pip install --upgrade six enum34 protobuf
+pip_install --upgrade pip==19.3.1
+pip_install --upgrade setuptools
+pip_install --upgrade cython
+pip_install --upgrade six enum34 protobuf
 
 if [ "$("$VENV_PYTHON" -c "import sys; print(sys.version_info[0])")" == "2" ]
 then
-  $VENV_PYTHON -m pip install futures
+  pip_install futures
 fi
 
 pip_install_dir "$ROOT"
@@ -214,7 +223,7 @@ pip_install_dir "$ROOT/src/python/grpcio_status"
 pip_install_dir "$ROOT/src/python/grpcio_testing"
 
 # Build/install tests
-$VENV_PYTHON -m pip install coverage==4.4 oauth2client==4.1.0 \
+pip_install coverage==4.4 oauth2client==4.1.0 \
                             google-auth==1.0.0 requests==2.14.2 \
                             googleapis-common-protos==1.5.5
 $VENV_PYTHON "$ROOT/src/python/grpcio_tests/setup.py" preprocess