Since fish 3.10 at least, the current way of loading rbenv in `fish` is misbehaving, at least in some environments:
```
source: Error encountered while sourcing file '/var/folders/pj/jn249gcn7ddfrjzj2_9mxjhw0000gp/T//.psub.f0iJSWRByB':
source: No such file or directory
```
This changes the initialization to the method recommended by a `fish` developer here: https://github.com/fish-shell/fish-shell/issues/6613#issuecomment-586679958
19ae300eb8