This PR fixes a bug found by @vade here, wherein the xcodebuild command to build the xcframework was being given the wrong paths to the constituent frameworks. It was introduced in 69e1167.

I also found another issue that prevents the use of absolute paths when assembling the xcframework, so I've also included a fix for that.

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or other license that is incompatible with OpenCV
• The PR is proposed to proper branch
• There is reference to original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake

force_builders_only=docs,ios,Custom Mac
build_image:Custom Mac=osx_framework-test
buildworker:Custom Mac=macosx-2