feat: Use JJ_LOG instead of RUST_LOG for tracing configuration

[theduke]

Prevents clashes when working on other Rust projects.

Closes #4219

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[theduke]

This should probably be documented somewhere, but not sure where the best place is.

[martinvonz]

This should probably be documented somewhere, but not sure where the best place is.

Perhaps docs/contributing.md is the best place we have right now

[thoughtpolice]

I think this would be nice to get in before the 0.21 release, which will happen tomorrow. @theduke Can you squash these commits and re-push? If not I'm happy to do so (and retain your authorship!) and refile them myself, since we disallow the "Squash" button on GitHub.

[samueltardieu]

Hi @theduke. Do you still plan to work on this?

[theduke]

Sorry I dropped the ball here, will get it fixed today.

[arxanas]

Superseded by https://github.com/martinvonz/jj/pull/5112.