utils: Use u8path to fix possible string conversion crashes on Windows

[PatTheMav]

Description

Use u8path when creating a file system path via a provided std::string.

Motivation and Context

All other path operations in the code base already use u8path to ensure that the strings provided by libobs (which will either be ASCII or UTF-8 byte sequences) are correctly converted into UTF-16 strings.

Fixes #1244.

How Has This Been Tested?

Has not been tested locally, will need simple reproducer first.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have read the Contributing Guidelines.
• All commit messages are properly formatted and commits squashed where appropriate.
• My code is not on master or a release/* branch.
• The code has been tested.
• I have included updates to all appropriate documentation.