test: prefer system pgrep/pkill to avoid proctools conflict

When `proctools` is installed, `brew test` fails with a SIGTERM during cleanup.
This happens because `proctools` provides `pgrep`/`pkill` binaries in `HOMEBREW_PREFIX/bin`,
which behave differently than system binaries on macOS.

This fix forces the use of system binaries in `/usr/bin` within the `system` call.

Fixes #21242