Guard pip install with flock

The tests run multiple instances, each instance using a different
virtual environment.

The issue looks similar to this pypa/pip#2361

It seems there are still issues with concurrent pip runs.

Added flock to guard all pip executions.

Fixes hazelcast#22599

extensions/python/src/main/resources/jet_to_python_init.sh

@@ -5,8 +5,10 @@ set -e -x
 
 python3 -m venv jet_to_python_env --system-site-packages
 source jet_to_python_env/bin/activate
-python3 -m pip install --upgrade pip
-python3 -m pip install protobuf==@protobuf.version@ grpcio==@grpc.version@
+
+flock --exclusive /tmp/jet-pip.lock python3 -m pip install --upgrade pip
+flock --exclusive /tmp/jet-pip.lock python3 -m pip install protobuf==@protobuf.version@ grpcio==@grpc.version@
+
 [[ -f requirements.txt ]] && python3 -m pip install -r requirements.txt
 source ./jet_to_python_params.sh
 if [[ -f init.sh ]]; then