Rollup merge of #125355 - michaelwoerister:rust_log_force_capture, r=nnethercote

Use Backtrace::force_capture instead of Backtrace::capture in rustc_log

After rust-lang/rust#125063, the compiler and custom drivers won't automatically set the RUST_BACKTRACE environment variable anymore, so we have to call `Backtrace::force_capture` instead of `Backtrace::capture` to unconditionally capture a backtrace.

rustc_log handles enabling backtraces via env vars itself, so we don't want RUST_BACKTRACE to make a difference.

Author: fmease