Delayed CTFE backtraces
This renames the env var that controls CTFE backtraces from `MIRI_BACKTRACE` to `RUST_CTFE_BACKTRACE` so that we can use `MIRI_BACKTRACE` in the miri tool to only show backtraces of the main miri execution.
It also makes `RUST_CTFE_BACKTRACE` only show backtraces that actually get rendered as errors, instead of showing them eagerly when the `Err` happens. The current behavior is near useless in miri because it shows about one gazillion backtraces for errors that we later catch and do not care about. However, @oli-obk likes the current behavior for rustc CTFE work so it is still available via `RUST_CTFE_BACKTRACE=immediate`.
NOTE: This is based on top of https://github.com/rust-lang/rust/pull/53821. Only [the last three commits](https://github.com/oli-obk/rust/compare/sanity_query...RalfJung:ctfe-backtrace) are new.
Fixes https://github.com/rust-lang/rust/issues/53355