Add a SPMI collection for pre-AVX512 hardware

Our current SPMI collection runs are all on CI machines with AVX-512 support and therefore we are missing coverage for the more common hardware that only has up through AVX2.

We should add a collection job that sets DOTNET_EnableAVX512F=0 to ensure we have sufficient coverage of that scenario.

We should already be getting coverage for DOTNET_EnableAVX=0 via the crossgen2 job and so likely do not need to add coverage for that as well.