fix unbundled build with pyside6

[chayleaf]

Your checklist for this pull request

• I've read the guidelines for contributing to this repository
• I made sure to follow the project's coding style
• I've updated the documentation with the relevant information (if needed)

Detailed description

This fixes 2 issues with the build script.

1. BUILD_INTERFACE and INSTALL_INTERFACE weren't being expanded in this particular case (for SHIBOKEN_INCLUDE_DIRS), which broke build, so I simply changed it to reference the directories directly.
2. Without bundled Rizin, rz_core wasn't in SHIBOKEN_INCLUDE_DIRS. This adds code to remedy this issue.

Test plan (required)

Make sure this builds with Pyside 6 and without bundling Rizin. Personally I used the following flags: -DCUTTER_USE_BUNDLED_RIZIN=OFF -DCUTTER_ENABLE_PYTHON=ON -DCUTTER_ENABLE_PYTHON_BINDINGS=ON -DCUTTER_ENABLE_GRAPHVIZ=ON -DCUTTER_QT6=ON

Note that if you have most stuff in /usr or /usr/local, the issue might not have appeared on your system at all even before this commit. In my case, it's necessary for building Cutter on NixOS: NixOS/nixpkgs#268874.

Closing issues

[karliss]

@chayleaf can you try the code from #3268 I don't have a NixOS enviornment set up

Didn't yet try to with nonbundled rizin, but I have a feeling that the whole copy pasted header handling part which was introduced in #2952 needs to be removed.

[chayleaf]

closing in favor of #3268