Panics don't lock stderr while printing panic info

Panics don't seem to lock stderr while printing, so if multiple threads panic at once, this will happen:

thread 'thread 'thread 'thread 'State runner 5 (sync)State runner 1 (sync)State runner 0 (sync)State runner 4 (sync)' panicked at ' panicked at ' panicked at ' panicked at src/circuits/relay.rssrc/circuits/relay.rssrc/circuits/relay.rssrc/circuits/relay.rs::::167167167167::9::9:
99:
not yet implemented:
:
not yet implemented
not yet implementednot yet implemented
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

As you can see, i forgot a todo!() in the code and 4 threads executed it at the same time

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rustc --version --verbose:

rustc 1.79.0-nightly (ccfcd950b 2024-04-15)
binary: rustc
commit-hash: ccfcd950b333fed046275dd8d54fe736ca498aa7
commit-date: 2024-04-15
host: x86_64-unknown-linux-gnu
release: 1.79.0-nightly
LLVM version: 18.1.3

Start by reproducing the interleaved output from the report with multiple threads panicking simultaneously, using the mentioned Rust nightly version and RUST_BACKTRACE=1 if needed. Then trace rustc's panic-reporting and stderr output paths; done means concurrent panic messages are serialized and covered by an appropriate regression test.