With the new update to 1.2 in https://github.com/NixOS/nixpkgs/pull/79426, some
the install tree coming out of cmake seems to have produced a duplicate copy of
the nix tree.
This commit continues the work proposed in #68014, and provides an entirely FOSS
variant of the `dnnl` package. Updates to add an `mkl` flavor will be added
later, pending discussion about the cleanest way to overlay this consistently.
Fixes#67982, closes#68014
Co-authored-by: Alex Rice <alexrice999@hotmail.co.uk>