[ci][framework] Don't unconditionally force clang-18 for UNIX platforms in CMakeLists.txt
- Related issues: ivas-codec-pc/ivas-codec#1591
- Requested reviewers: @kiene, @haeussler
Reason why this change is needed
Description of the change
- Added a guard around the compiler setting
Affected operating points
- CMake Build
Related to #1591 (closed)