buildscripts: xds-k8s pin pip to 21.0.1

pip 21.1 released on Apr 24 introduced a regression for python 3.6.1.
The regression was identified on Apr 24, the fix merged on Apr 25.
The fix is expected to be delivered in the 21.1.1 patch.

There's no clear date, when 21.1.1 will be released.
Until then, pin is temporarily pinned to the previous release, 21.0.1.

Author: sergiitk

buildscripts/kokoro/xds-k8s-install-test-driver.sh

@@ -256,9 +256,12 @@ kokoro_setup_python_virtual_environment() {
   eval "$(pyenv virtualenv-init -)"
   py_latest_patch="$(pyenv versions --bare --skip-aliases | grep -E "^${PYTHON_VERSION}\.[0-9]{1,2}$" | sort --version-sort | tail -n 1)"
   echo "Activating python ${py_latest_patch} virtual environment"
-  pyenv virtualenv "${py_latest_patch}" k8s_xds_test_runner
+  pyenv virtualenv --no-pip "${py_latest_patch}" k8s_xds_test_runner
   pyenv local k8s_xds_test_runner
   pyenv activate k8s_xds_test_runner
+  python -m ensurepip
+  python -m pip install -U pip==21.0.1
+  pip --version
 }
 
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