The repository information for cloning the OpenXR repository, rather than just
downloading the source archive, was outdated.
- Use the OpenXR-SDK repository link, not the OpenXR-SDK-Source one. Only the former is needed, it contains pregenerated files with minimum dependencies. The latter contains additional tools and the un-generated source files.
- Update the commit hash to the version we currently use, 1.0.14 (was 1.0.6).