Especially older hardware doesn't support AVX instructions. DLib is
still functional there, but significantly slower[1].

By setting `avxInstructions` to false, DLib will be compiled without
this feature.

[1] http://dlib.net/compile.html

There's no git tag for 1.2.3, hence we need to pin to the corresponding
revision because we build from a git source.

After recent breakage on Hydra[1], the tests were disabled. Although
some build machines don't support AVX, we shouldn't use a DLib without
AVX as the builder's result is also used on modern machines with AVX
support. Before merging changes, maintainers should run the check phase
locally in a `nix-shell`.

[1] https://hydra.nixos.org/build/89533530

dlib: improve AVX configuration