Add environment variable to change Jetstream version

[vivianrwu]

Description

Adds an environment variable to change JetStream version in Docker builds in case JetStream repository is broken at HEAD.

Tests

Manually tested by building the Dockerfile and testing the maxengine_server.

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed.