check_cpu_feature_support makes repeated/unnecessary calls to std::getenv("XSS_DISABLE_AVX512")

[jcarreira]

The result of std::getenv("XSS_DISABLE_AVX512") should be cached in lib/x86simdsort.cpp::check_cpu_feature_support .

[r-devulap]

On what OS are you running it? I presume windows/MSVC, because on linux, check_cpu_feature_support should only be triggered once at load time.