Use user installation for pip and nox

Change-Id: I84f46760b7e221459331eaed30416bd04114fd19

Author: Jon Wayne Parrott

.kokoro/system_tests.sh

@@ -27,9 +27,9 @@ export GOOGLE_APPLICATION_CREDENTIALS=$(pwd)/testing/service-account.json
 export GOOGLE_CLIENT_SECRETS=$(pwd)/testing/client-secrets.json
 
 # Run tests
-pip install --upgrade pip
-pip install --upgrade nox-automation
-nox -k "${NOX_SESSION}" || ret_code=$?
+pip install --user --upgrade pip
+~/.local/bin/pip install --user --upgrade nox-automation
+~/.local/bin/nox -k "${NOX_SESSION}" || ret_code=$?
 
 # Workaround for Kokoro permissions issue: delete secrets
 rm testing/{test-env.sh,client-secrets.json,service-account.json}