Saf2 gh 765 add env check target

Both `can_execute` and `check_file` return if it's *broken*, but their function names are worded as if they check if it's functioning. E.g. if pip works, can_execute returns `False`, whereas the function name (and the way you're using it here) suggest that it ought to be `True`. I suggest inverting the logic in both of those functions.