[ci][framework] Don't unconditionally force clang-18 for UNIX platforms in CMakeLists.txt

Archit Tamarapurequested to merge
float-1591-cmakelists-txt-overrides-cmake_c_compiler-forcing-clang-18-when-ivas-is-included-as-a into main

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

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