Reconsider stracktrace configurability with rust 2024 set_var changes.

I know there has already been a lot of discussion on this, so my apologies for bringing it up again, but rust 2024 will change the original context this decision may have been made in.

Previously I raised https://github.com/dtolnay/anyhow/issues/342 but then I found a workaround to disable anyhow backtrace capturing via:
```rust
if std::env::var("RUST_LIB_BACKTRACE").is_err() {
std::env::set_var("RUST_LIB_BACKTRACE", "0");
}
```
This resolved my issue and the issue was closed.

However, with rust 2024, [set_var will be marked unsafe](https://doc.rust-lang.org/nightly/edition-guide/rust-2024/newly-unsafe-functions.html).
Many high level codebases have a blanket ban on unsafe, so even if `set_var` can be soundly called at the start of the application before any threads are created, there is no way to create a sound wrapper API for that as it could still be called after that point.
So this approach is not usable in codebases with a blanket ban on unsafe.

Does this alter the original assumptions around the decision to "not expose a way for the application to configure whether stacktraces are captured"?
If this changes nothing go ahead and close the issue.