Page shifts when opening the Search modal when using scrollbar-gutter

## Description

When the page uses `scrollbar-gutter`, the right margin fix introduced in [v3.8.3](https://github.com/algolia/docsearch/releases/tag/v3.8.3) doesn't account for this.

This leads to extra spacing, effectively reintroducing the issue for users utilizing `scrollbar-gutter`.

## Steps to reproduce

### For Windows

* Visit the https://fylgja.dev/library/base/
* Click on search bar
* Observe how the elements beneath the popup modal shift to the right.

### For Mac

* Change the scrollbar behavior to always visible.
* Visit the https://fylgja.dev/library/base/
* Click on search bar
* Observe how the elements beneath the popup modal shift to the right.

## Expected behavior

Only apply the margin right fix if the page is not using `scrollbar-gutter` on the html tag or make this margin right fix optional

## Environment

- OS: Windows / Linux / macOS
- Browser: All browsers
- DocSearch version: 3.8.3 and newer

## Relates to

* https://github.com/algolia/docsearch/issues/2378
* https://github.com/algolia/docsearch/pull/2413

Reproduce the shift on the fylgja.dev page with scrollbar-gutter enabled, then review the v3.8.3 change and the related issue 2378 and pull request 2413 to locate the margin-right fix. Done means the modal opens without shifting underlying elements when scrollbar-gutter is present, while the existing behavior remains correct otherwise.

Written by the indexing model from the issue text.