[Fix] Auto-detect Python executable in cmake utils

[YangKai0616]

run_python(), run_python_script() and try_run_python() in utils.cmake previously hardcoded Python_EXECUTABLE or Python3_EXECUTABLE.

For example, this causes get_gpu_lang.py to fail with "Cannot detect GPU language" when building kernels locally via pip install -e ..

The error message is as follows:

...
running build_py
      running build_ext
      -- The CXX compiler identification is IntelLLVM 2025.3.2
      -- Detecting CXX compiler ABI info
      -- Detecting CXX compiler ABI info - done
      -- Check for working CXX compiler: /opt/intel/oneapi/compiler/2025.3/bin/icpx - skipped
      -- Detecting CXX compile features
      -- Detecting CXX compile features - done
      -- FetchContent base directory: /tmp/tmp5jy45gzj.build-temp/flash_attn2._flash_attn2_xpu_37cbe71_dirty/_deps
      -- Found Python3: /opt/venv/bin/python (found version "3.12.3") found components: Development Development.SABIModule Interpreter Development.Module Development.Embed
      CMake Error at cmake/utils.cmake:60 (message):
        Cannot detect GPU language:
      Call Stack (most recent call first):
        cmake/get_gpu_lang.cmake:5 (run_python_script)
        CMakeLists.txt:41 (get_gpu_lang)
...

This PR makes all three functions auto-detect the available Python executable.

[YangKai0616]

Hi @danieldk , @drbh , pls help review. Thanks!

[YangKai0616]

Hey @danieldk , we can re-trigger the CI. I’ve already tested both the local build and the nix build successfully.

[danieldk]

Thanks a lot! Test failures are unrelated.