Reserve RUSTUP_LOG for tracing usages instead of RUST_LOG

[rami3l]

Problem you are trying to solve

@weihanglo has raised concerns regarding RUST_LOG adding Rustup's log lines to cargo run when cargo is a rustup shim.

Cargo has changed to using CARGO_LOG for cargo-internal log lines for clarity in rust-lang/cargo#6918, and considering the special relationship between the two projects, I think it's reasonable for us to do the same.

Solution you'd like

Reserve RUSTUP_LOG for tracing usages inside of Rustup instead of the default RUST_LOG.

Notes

No response

[djc]

Yup, this makes sense to me.