flake.nix : rewrite (#4605)

flake.nix : rewrite (#4605)

* flake.lock: update to hotfix CUDA::cuda_driver

Required to support https://github.com/ggerganov/llama.cpp/pull/4606

* flake.nix: rewrite

1. Split into separate files per output.

2. Added overlays, so that this flake can be integrated into others.
   The names in the overlay are `llama-cpp`, `llama-cpp-opencl`,
   `llama-cpp-cuda`, and `llama-cpp-rocm` so that they fit into the
   broader set of Nix packages from [nixpkgs](https://github.com/nixos/nixpkgs).

3. Use [callPackage](https://summer.nixos.org/blog/callpackage-a-tool-for-the-lazy/)
   rather than `with pkgs;` so that there's dependency injection rather
   than dependency lookup.

4. Add a description and meta information for each package.
   The description includes a bit about what's trying to accelerate each one.

5. Use specific CUDA packages instead of cudatoolkit on the advice of SomeoneSerge.

6. Format with `serokell/nixfmt` for a consistent style.

7. Update `flake.lock` with the latest goods.

* flake.nix: use finalPackage instead of passing it manually

* nix: unclutter darwin support

* nix: pass most darwin frameworks unconditionally

...for simplicity

* *.nix: nixfmt

nix shell github:piegamesde/nixfmt/rfc101-style --command \
    nixfmt flake.nix .devops/nix/*.nix

* flake.nix: add maintainers

* nix: move meta down to follow Nixpkgs style more closely

* nix: add missing meta attributes

nix: clarify the interpretation of meta.maintainers

nix: clarify the meaning of "broken" and "badPlatforms"

nix: passthru: expose the use* flags for inspection

E.g.:

```
❯ nix eval .#cuda.useCuda
true
```

* flake.nix: avoid re-evaluating nixpkgs too many times

* flake.nix: use flake-parts

* nix: migrate to pname+version

* flake.nix: overlay: expose both the namespace and the default attribute

* ci: add the (Nix) flakestry workflow

* nix: cmakeFlags: explicit OFF bools

* nix: cuda: reduce runtime closure

* nix: fewer rebuilds

* nix: respect config.cudaCapabilities

* nix: add the impure driver's location to the DT_RUNPATHs

* nix: clean sources more thoroughly

...this way outPaths change less frequently,
and so there are fewer rebuilds

* nix: explicit mpi support

* nix: explicit jetson support

* flake.nix: darwin: only expose the default

---------

Co-authored-by: Someone Serge <redacted>