integration-tests: force pip installation

Alpine Linux now complains that:

    The system-wide python installation should be maintained using the system
    package manager (apk) only.

But lbpytest is not available in the Alpine repositories, so we *have*
to install it via pip.

Using a virtual environment -- as suggested -- does not work either, we
really want to install it globally.

So use a workaround to override this check.

integration-tests/Dockerfile

@@ -3,7 +3,7 @@ FROM alpine
 RUN apk update && \
     apk add --no-cache openssh bash python3 py3-pip
 
-RUN pip install lbpytest
+RUN pip install --break-system-packages lbpytest
 
 COPY entry.sh gatewaytest.py /
 COPY tests /tests