Rollup merge of #107761 - oli-obk:miri_🪵, r=TaKO8Ki

Replace a command line flag with an env var to allow tools to initialize the tracing loggers at their own discretion

fixes https://github.com/rust-lang/miri/issues/2778

this was introduced in https://github.com/rust-lang/rust/pull/104645, so this PR reverts the flag-part and uses an env var instead.

diff --cc compiler/rustc_driver_impl/src/lib.rs
Simple merge