ScanByte issue with packageimages

[jakobnissen]

Dear mantainers,

I just found a bug in ScanByte with how it interacts with package images: If the user sets export JULIA_CPU_TARGET=generic, then ScanByte will still emit code for e.g. AVX2, but then the package image will be broken as Julia cannot use the generated instructions.
I fixed this in ScanByte v 0.4.0, but I recommend just doing what I do and check if ENV["JULIA_CPU_TARGET"] is set to something other than "native", and avoid platform-specific code if so.