LINALG_HAS_EXECUTION not strict enough for some clang compilers?

[amklinv-nnl]

See https://github.com/amklinv-nnl/stdBLAS/actions/runs/27846446639/job/82416543257#logs for details. This test guards all references to std::execution with LINALG_HAS_EXECUTION, which doesn't appear to be strict enough for certain clang compilers.

/Users/runner/work/stdBLAS/stdBLAS/tests/native/gemv_no_ambig.cpp:47:43: error: no member named 'par' in namespace 'std::execution'
47 | matrix_vector_product(std::execution::par,

[amklinv-nnl]

I'll add a CMake check for this. I have to do it to get coverage working anyway. Feel free to "assign" me if you like.