The previous version rebound the `hash` and `rehash` builtins to
reassing GNU utils when they were called. This didn't take into
account that when `$PATH` changes, the commands are rehashed as
well.

This version adds a preexec hook to reassign the GNU utilities
before running any command, if and only if they aren't already
reassigned. This is checked by looking at the `whoami` command.

Fixes #10535