Enable the Synapse Python Client to read PAT from env var

[tschaffter]

Bruce told me today that a recent update enables the Synapse Python client to read the personal access token from an env var (TODO: identify the name of this env var). This way we would no longer need to create the Synapse config file when the container starts, which is more secure, for example if the developer takes a snapshot of the container and push it to a Docker registry.