Fix JtR::Cracker's ability to find shipped bins

### Verification
 *On a system for which Framework ships `john` binaries, i.e. Linux (x86 or x64) or Windows*:
 - [x] `which john`
   - [x] **VERIFY** no john in your path
 - [x] msfconsole
 - [x] Get a system session on a windows box
 - [x] `use post/windows/gather/hashdump`
 - [x] `set SESSION 1`
 - [x] `run`
 - [x] `creds`
   - [x] **VERIFY** hashes show up
 - [x] `use auxiliary/analyze/jtr_crack_fast`
 - [x] `show options`
   - [x] **VERIFY** you have *not* set JOHN_PATH (`unset JOHN_PATH` and/or `unsetg JOHN_PATH` if you have)
 - [x] `run`
   - [x] **VERIFY** no stack trace
   - [x] **VERIFY** it takes no more than about 30 seconds