completions/vertico sets `completion-category-overrides` instead of modifying it

### I confirm that...

- [X] I have searched the [issue tracker](https://github.com/doomemacs/doomemacs/issues), [documentation](https://docs.doomemacs.org), [FAQ](https://docs.doomemacs.org/-/faq), [Discourse](https://discourse.doomemacs.org), and [Google](https://google.com), in case this issue has already been reported/resolved.

- [X] I have read "[How to Debug Issues](https://doomemacs.org/d/how2debug)", and will use it to provide as much information about this issue as possible.

- [X] The issue can be reproduced on the **latest** available commit of Doom.

- [X] The issue can be reproduced on a stable release of Emacs, such as 27, 28, or 29. *(Unstable versions end in .50, .60, or .9x)*

### Expected behavior

The vertico completion module should modify the `completion-category-overrides` alist with new overrides for file.

### Current behavior

The vertico completion completely overwites the `completion-category-overrides` alist so that any previous values are lost.

### Steps to reproduce

1. Enable `completions/vertico`

### System Information

https://gist.github.com/snyball/0dfff215723f25dd07871c79158afe7a