Keep init-file when using LEIN_FAST_TRAMPOLINE

Otherwise the memoized startup will fail because of the missing init-file:

```
→ lein trampoline test
Exception in thread "main" java.io.FileNotFoundException: /tmp/form-init5088991943718769025.clj (Datei oder Verzeichnis nicht gefunden)
```

Still an issue: The init-file will most likely be deleted on reboot.