Add workflow to include pre-built loader on macOS

[xwovr]

Here is an example run which triggered manually:
https://github.com/xwovr/OpenXR-SDK-Source/actions/runs/12325232718

Test steps:

1. Download the artifact openxr_loader_macos.zip from the above run
2. Unzip the artifact to openxr_loader_macos folder. And then cd openxr_loader_macos
3. Following the instruction in README.md, run xattr -d com.apple.quarantine lib/libopenxr_loader.dylib
4. Create new Unity project, import com.unity.xr.openxr, com.unity.xr.interaction.toolkit and com.meta.xr.simulator packages
5. Replace the libopenxr_loader.dylib in the project's "Packages/OpenXR Plugin/RuntimeLoaders/osx" folder with the one downloaded
6. Meta XR Simulator can be activated and launched successfully using the new OpenXR loader

[CLAassistant]

All committers have signed the CLA.

[rpavlik-bot]

An issue (number 2427) has been filed to correspond to this pull request in the internal Khronos GitLab (Khronos members only: KHR:openxr/openxr#2427 ), to facilitate working group processes.

This GitHub pull request will continue to be the main site of discussion.