Explicitly testing generic and non-generic code variants

[praveksharma]

As pointed out in https://github.com/open-quantum-safe/liboqs/pull/1745#discussion_r1702849222 CI does not explicitly use the OQS_USE_CPUFEATURE_INSTRUCTION build flag to test generic and non-generic code variants right now.

The generic tests in weekly.yml do test generic code by specifying the CPU architecture to the compiler and GitHub hosted runners seem to have the required CPU extensions to run and test AVX2 code (for instance, see https://github.com/open-quantum-safe/liboqs/actions/runs/10358446454/job/28672828780 under Configure) in the linux tests in unix.yml --- so both generic and non-generic code do get tested.

Making these tests explicit by using the OQS_USE_CPUFEATURE_INSTRUCTION flag is desirable because the documentation for GitHub hosted runners does not guarantee that these runners support specific CPU flags.