Cargo.toml: allow "fuzzing" as `cfg` condition name

[cakebaker]

This PR allows "fuzzing" as a cfg condition name in order to fix the "unexpected cfg condition name: fuzzing" warnings on the nightly builds in the CI (see, for example, https://github.com/uutils/coreutils/actions/runs/9673817439/job/26688539353?pr=6496#step:7:504).