Does not respect JULIA_CPU_TARGET

MWE:
```
$ export JULIA_CPU_TARGET=generic
$ julia --startup-file=no -q

julia> using HostCPUFeatures

julia> has_feature(Val{:x86_64_avx2}())
static(true)
```
This cause issues during precompilation with pkgimages on Julia 1.9, e.g. https://github.com/BioJulia/FASTX.jl/issues/101.

Without knowing many of the details, I think this should be relatively easy to fix. Setting JULIA_CPU_FEATURES forces precompilation of all packages, so HostCPUFeatures can simply check at precompile time if the env variable is set, and then adjust its compile time results accordingly.