Try to configure Github Actions to actually use the SIMD backend

[hdevalence]

The GH Actions added in #3 don't actually check the simd backend, because they're compiled without awareness of the CPU flags that would support AVX2, and the implementation doesn't do dynamic dispatch. Two ways to address this:

1. Set the target_cpu to native and hope that actions runs on something with AVX2 (likely, since AVX2 is not that new);
2. Use qemu or something to run the avx2 or ifma implementations in an emulator;

[saleemrashid]

Could you use -C target-feature=+avx2 and then you'd hit an illegal instruction in the unlikely case that the runner doesn't support AVX2