Add user journey test to retrieve a lockfile for a build

[peytondmurray]

Fixes #673.

Description

This PR adds a user journey test to add a lockfile for a build. Depends on #804, so I'll mark this as a draft until that one is merged.

This pull request:

• Adds a user journey test for getting a lockfile for a successful build as an admin user, and checking that it contains the expected dependencies.
• Adds a function to api_utils.py to retrieve the lockfile from the conda-store server

Pull request checklist

• Did you test this change locally?
• Did you update the documentation (if required)?
• Did you add/update relevant tests for this change (if required)?

How to test

Run the conda-store server:

docker compose up

Then run the test:

cd conda-store-server
pytest tests/user_journeys/test_user_journeys.py::test_get_lockfile

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[dcmcand]

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