logging: add env var to control verbose scope entry/exit logging
~~This got removed in https://github.com/rust-lang/rust/pull/75143, and I find this makes long traces a lot harder to read, so I propose we add this back.~~
Example trace:
```
│ │ ├─0ms INFO rustc_const_eval::interpret::step return
│ │ ├─0ms INFO rustc_const_eval::interpret::eval_context popping stack frame (returning from function)
│ │┌┘rustc_const_eval::interpret::eval_context::frame std::ptr::mut_ptr::<impl *mut u8>::guaranteed_eq
│ ├┘rustc_const_eval::interpret::eval_context::frame std::ptr::mut_ptr::<impl *mut u8>::is_null
│ ├─1ms INFO rustc_const_eval::interpret::step // executing bb2
│ ├─1ms INFO rustc_const_eval::interpret::step StorageDead(_4)
│ ├─1ms INFO rustc_const_eval::interpret::step StorageDead(_2)
│ ├─1ms INFO rustc_const_eval::interpret::step return
│ ├─1ms INFO rustc_const_eval::interpret::eval_context popping stack frame (returning from function)
│┌┘rustc_const_eval::interpret::eval_context::frame std::ptr::mut_ptr::<impl *mut u8>::is_null
├┘rustc_const_eval::interpret::eval_context::frame std::sys_common::thread_local_dtor::register_dtor_fallback::run_dtors
├─178ms INFO rustc_const_eval::interpret::step // executing bb2
├─178ms INFO rustc_const_eval::interpret::step StorageDead(_5)
```
r? `@oli-obk`
afb9171b64