Disable CK Builder for SLES15 in Jenkins CI (#3581)

1. Added `-DCK_EXPERIMENTAL_BUILDER=OFF` to the `setup_args` to explicitly disable the experimental builder

2. Added a detailed comment explaining why this is necessary:

   - SLES15 is a legacy platform with limited C++20 ecosystem support
   - While the ROCm compiler supports C++20, the older system libraries and standard library implementation on SLES15 does not reliably support all C++20 features required by the experimental CK Builder