ci/eval: allow precisely choosing which systems to evaluate for (evalSystem ->...

Furthermore it also allows local evaluation using

```

nix-build ci -A eval.full \

  --max-jobs 4

  --cores 2

  --arg chunkSize 10000

  --max-jobs 4 \

  --cores 2 \

  --arg chunkSize 10000 \

  --arg evalSystems '["x86_64-linux" "aarch64-darwin"]'

```

- `--max-jobs`: The maximum number of derivations to run at the same time. Only each [supported system](../supportedSystems.nix) gets a separate derivation, so it doesn't make sense to set this higher than that number.

- `--cores`: The number of cores to use for each job. Recommended to set this to the amount of cores on your system divided by `--max-jobs`.

- `chunkSize`: The number of attributes that are evaluated simultaneously on a single core. Lowering this decreases memory usage at the cost of increased evaluation time. If this is too high, there won't be enough chunks to process them in parallel, and will also increase evaluation time.

- `evalSystems`: The set of systems for which `nixpkgs` should be evaluated. Defaults to the four official platforms (`x86_64-linux`, `aarch64-linux`, `x86_64-darwin` and `aarch64-darwin`).

A good default is to set `chunkSize` to 10000, which leads to about 3.6GB max memory usage per core, so suitable for fully utilising machines with 4 cores and 16GB memory, 8 cores and 32GB memory or 16 cores and 64GB memory.

  full =

    {

      # Whether to evaluate just a single system, by default all are evaluated

      evalSystem ? if quickTest then "x86_64-linux" else null,

      # Whether to evaluate on a specific set of systems, by default all are evaluated

      evalSystems ? if quickTest then [ "x86_64-linux" ] else supportedSystems,

      # The number of attributes per chunk, see ./README.md for more info.

      chunkSize,

      quickTest ? false,

    }:

    let

      systems = if evalSystem == null then supportedSystems else [ evalSystem ];

      results = linkFarm "results" (

        map (evalSystem: {

          name = evalSystem;

            inherit quickTest evalSystem chunkSize;

            attrpathFile = attrpathsSuperset + "/paths.json";

          };

        }) systems

        }) evalSystems

      );

    in

    combine {