fix pipeline (#1723)

Author: minhthuc2502

python/tools/prepare_test_environment.sh

@@ -4,6 +4,7 @@ set -e
 set -x
 
 # Install test rquirements
+pip cache purge
 pip --no-cache-dir install -r python/tests/requirements.txt --extra-index-url https://download.pytorch.org/whl/cpu
 pip uninstall -y ctranslate2