Merge pull request #15080 from matt-kwong/pythonpath

Upgrade pip for virtualenv Python in build_python.sh

tools/run_tests/helper_scripts/build_python.sh

@@ -142,6 +142,11 @@ fi
  true)
 VENV_PYTHON=$(script_realpath "$VENV/$VENV_RELATIVE_PYTHON")
 
+# See https://github.com/grpc/grpc/issues/14815 for more context. We cannot rely
+# on pip to upgrade itself because if pip is too old, it may not have the required
+# TLS version to run `pip install`.
+curl https://bootstrap.pypa.io/get-pip.py | $VENV_PYTHON
+
 # pip-installs the directory specified. Used because on MSYS the vanilla Windows
 # Python gets confused when parsing paths.
 pip_install_dir() {