Add `conda config` warning docs/message; add duration log for actions

[peytondmurray]

Fixes #810.

Description

This PR adds:

1. A warning message about environment variables used by conda-store to override user conda configuration, which should help clear up user uncertainty about the output of conda config.
2. Some information in the documentation about the above :point_up:
3. An info-level log message is now emitted displaying the amount of time each action takes.

Pull request checklist

• [x] Did you test this change locally? Yes, but not all tests pass locally on main. The ones I expected to pass locally have passed.
• [x] Did you update the documentation (if required)?
• [x] Did you add/update relevant tests for this change (if required)?

Notes

• To see that this works locally, run any action. Alternatively, run pytest conda_store_server/test/test_actions.py::test_action_decorator - that test specifically checks for the run time information output to stdout that this PR implements.

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