report upstream: block/buzz test hard-codes /bin/kill #295

`desktop_provider::ownership::tests::proof_timeout_and_cancellation_close_the_socket_and_kill_lsof` (block/buzz `crates/buzz-cli/src/desktop_provider/ownership.rs`, added in block/buzz#9270) checks whether its child is still alive with `Command::new("/bin/kill")`. The Nix sandbox on Linux has no `/bin/kill` (NixOS has none either), so the test panics with `NotFound` (Buzz canary run https://github.com/kleinbem/nix-packages/actions/runs/42807713462, #291).

The Buzz canary substitutes util-linux's `kill` for now. Upstream could use `kill` from `PATH`, or `libc::kill(pid, 0)` / `nix::sys::signal::kill`, which needs no binary at all. The nixpkgs recipe (buzz-next) needs the same substitution once its Buzz version has the test.