Fix Python virtual environments in Dockerfiles

[data-sync-user]

In some of our Dockerfiles, we don’t correctly manage Python virtual environments. This hasn’t caused any issues so far, but it would be good to get those updated.

This file uses the correct method: https://github.com/mozilla-services/merino/blob/main/test-engineering/contract-tests/kinto-setup/Dockerfile

This is an example for a file that is not using the correct method: https://github.com/mozilla-services/merino/blob/main/test-engineering/contract-tests/client/Dockerfile

Here’s another one: https://github.com/mozilla-services/contile-integration-tests/blob/main/client/Dockerfile

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