This generates profiles in the Google Chrome JSON tracing format. They can be opened in Chrome's `chrome://tracing` page or in tools like https://ui.perfetto.dev. Enable by running e.g. `JJ_TRACE=1 cargo run -- status`.
This gets used by `nix develop`, or automatically by `direnv` if you have it
installed.
The binary versions are pinned to the versions recommended by `contributing.md`.
```
>> cargo --version
cargo 1.60.0 (d1fd9fe 2022-03-01)
>> rustc --version
rustc 1.60.0 (7737e0b5c 2022-04-04)
>> cargo fmt --version
rustfmt 1.5.1-nightly (3984bc5 2023-01-17)
>> cargo clippy --version
clippy 0.1.60 (7737e0b 2022-04-04)
```
