Bisect with `--timeout` panics on Mac

Trying to use the --timeout flag on Mac causes the bisector to panic:

$ RUST_BACKTRACE=1 cargo-bisect-rustc --start=2022-10-03 --preserve --timeout 5

checking the start range to find a passing nightly
installing nightly-2022-10-03
testing...
thread 'main' panicked at 'thiserror::Errored to run "timeout" "5" "cargo" "+bisector-nightly-2022-10-03-x86_64-apple-darwin" "build": Os { code: 2, kind: NotFound, message: "No such file or directory" }', /Users/*/.cargo/registry/src/github.com-1ecc6299db9ec823/cargo-bisect-rustc-0.6.4/src/toolchains.rs:302:17
stack backtrace:
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

I believe cargo-bisect-rustc is assuming the presence of a utility called timeout that is common on Linux but not on Mac. The --timeout feature was added in PR #135 (@savente93, @ehuss).

Version

$ cargo-bisect-rustc --version
cargo-bisect-rustc 0.6.4

Start by inspecting the timeout handling associated with toolchains.rs:302 and the implementation introduced in PR #135. Reproduce the command on macOS, then verify that using --timeout no longer panics when the timeout utility is unavailable and that the bisect command still respects the requested timeout.