UI: Fix broken module paths provided via environment on macOS

Description

Fix macOS-specific paths added to list of module paths when custom environment variables are provided to OBS Studio at launch.

Motivation and Context

When custom module and module data paths are provided via environment variables, the module paths generated by current code will not be compatible with the plugin bundle structure used on macOS.

Using environment variables is a simple way to test plugins, as this allows OBS to discover them from alternative user-provided locations.

How Has This Been Tested?

Tested on macOS with plugins put into a custom location and provided the same directory as OBS_PLUGINS_PATH and OBS_PLUGINS_DATA_PATH via Xcode launch options.

Types of changes

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

Checklist:

[x] My code has been run through clang-format.
[x] I have read the contributing document.
[x] My code is not on the master branch.
[x] The code has been tested.
[x] All commit messages are properly formatted and commits squashed where appropriate.
[x] I have included updates to all appropriate documentation.

obsproject / obs-studio