search-in-workspace: fix padding when hovering over head node

[Archie27376]

What it does

Fixes #10363
The pull-request fixes an issue which causes content in the search-in-workspace tree nodes to shift when hovered. The fix ensures that the shift no longer happens by update the flex property for the button container.

How to test

• start the application, and open the search-in-workspace view
• perform a search which yields results
• confirm that hovering over nodes does not cause them to move

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[vince-fugnitto]

I confirmed that the behavior works as intended, and resolves the bug on master:

• no more shifting for standard search results
• styling looks correct when resized, when the replace icon is present
• styling looks correct when a single or multi-root workspace are used

[msujew]

The changes also look good to me, and I couldn't notice any regressions 👍