Rollup merge of #63440 - RalfJung:ctfe-backtrace, r=oli-obk
Rollup merge of #63440 - RalfJung:ctfe-backtrace, r=oli-obk

rename RUST_CTFE_BACKTRACE to RUSTC_CTFE_BACKTRACE

This matches `RUSTC_LOG`. Both affect only rustc, not any Rust program.

graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust