Remove intel mkl from flake.nix due to missing files

[Freed-Wu]

NixOS's mkl misses some libraries like mkl-sdl.pc. See #2261
Currently NixOS doesn't have intel C compiler (icx, icpx). See https://discourse.nixos.org/t/packaging-intel-math-kernel-libraries-mkl/975
So remove it from flake.nix

Some minor changes:

• Change pkgs.python310 to pkgs.python3 to keep latest
• Add pkgconfig to devShells.default
• Remove installPhase because we have cmake --install from Fix #2252, add install() to CMakeLists.txt #2256

Now build it for NixOS on x86_64-linux will use openBlas:

$ nix build --impure
$ tree result/
 result
├──  bin
│  ├──  baby-llama
│  ├──  benchmark
│  ├──  convert-lora-to-ggml.py
│  ├──  convert.py
│  ├──  embd-input-test
│  ├──  embedding
│  ├──  llama
│  ├──  llama-server
│  ├──  perplexity
│  ├──  quantize
│  ├──  quantize-stats
│  ├──  save-load-state
│  ├──  simple
│  ├──  test-grad0
│  ├──  test-quantize-fns
│  ├──  test-quantize-perf
│  ├──  test-sampling
│  ├──  test-tokenizer-0
│  └──  train-text-from-scratch
└──  lib
   ├──  libembdinput.so
   ├──  libggml_shared.so
   └──  libllama.so
$ ldd result/bin/llama | rg openblas
7:      libopenblas.so.0 => /nix/store/0cz1gdz17hk9myawjpycscm4dnhy7b3l-openblas-0.3.21/lib/libopenblas.so.0 (0x00007f2ea7860000)

I have tested it can work for NixOS on x86_64-linux.