Plugin zsh_reload: make sure cache dir exists

The [`zsh_reload`](https://github.com/robbyrussell/oh-my-zsh/tree/master/plugins/zsh_reload) plugin uses the `$ZSH/cache` directory, which does not exist on a fresh OMZ install. Hence the `src` function will fail to recompile the compdump:
```
re-compiling /home/ncanceill/.oh-my-zsh/cache/zcomp-epsilon.zwc: zrecompile:zcompile:133: can't write zwc file: /home/ncanceill/.oh-my-zsh/cache/zcomp-epsilon.zwc
re-compiling /home/ncanceill/.oh-my-zsh/cache/zcomp-epsilon.zwc: failed
```

This PR adds a check for it.

@mcornella @eMxyzptlk any objections?
Haha I thought I was more of a pain in the ass. In that case, the [`cache` variable could use a `local` before it](https://github.com/robbyrussell/oh-my-zsh/pull/2749#discussion_r12028761) :wink: