Changed RUST_LOG usage to CARGO_LOG to avoid confusion.

[zachlute]

Fixes #6189.

Changes Cargo to to initialize env_logger with the CARGO_LOG variable instead of RUST_LOG. Also updated documentation, tests, etc. to match the change.

[rust-highfive]

r? @matklad

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[alexcrichton]

Thanks for the PR! In light of https://internals.rust-lang.org/t/cargo-and-rustup-development/8615/1 I don't think we're willing to merge this at this time. This is changing a very longstanding interface in Cargo which I unfortunately think we don't have ti,e to prioritize and review until after the edition and planning.

[zachlute]

Yeah, no worries. It was a small change (from an input-work perspective) and I don't think it's high priority or anything that would warrant taking time away from larger issues right now.

[alexcrichton]

Ok sure thing, would you be ok closing this in the meantime?

[zachlute]

Yeah, there's still the issue to track the request so no reason this has to sit around taking up space.

[Eh2406]

@alexcrichton I see why this should not be merged during the features-freeze, but I am strongly in favor of this change in general.

[zachlute]

Reopening this now that we're past the feature freeze that prevented serious discussion initially.

[zachlute]

Ugh, I merged instead of rebased this because I'm a moron and forget how to git. I'll clean up the mess and probably just open another PR. Ignore me.