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*:
 - [ ] `which john`
   - [ ] **VERIFY** no john in your path
 - [ ] msfconsole
 - [ ] Get a system session on a windows box
 - [ ] `use post/windows/gather/hashdump`
 - [ ] `set SESSION 1`
 - [ ] `run`
 - [ ] `creds`
   - [ ] **VERIFY** hashes show up
 - [ ] `use auxiliary/analyze/jtr_crack_fast`
 - [ ] `show options`
   - [ ] **VERIFY** you have *not* set JOHN_PATH (`unset JOHN_PATH` and/or `unsetg JOHN_PATH` if you have)
 - [ ] `run`
   - [ ] **VERIFY** no stack trace
   - [ ] **VERIFY** it takes no more than about 30 seconds