Add nix flake bake command

[edolstra]

Motivation

Given a flake, nix flake bake produces a new "baked" flake consisting of the same outputs, but they're pre-evaluated, i.e. each output is a call to a function (builtins.fakeDerivation) that substitutes the output directly. Thus a baked flake is much faster since it does (almost) no evaluation.

Example usage:

# nix flake bake patchelf --dest-dir ./patchelf-baked

# nix flake show path:./patchelf-baked
path:/home/eelco/Dev/nix-master/build/patchelf-baked
├───checks
│   └───x86_64-linux
│       ├───build: CI test
│       └───pre-commit: CI test
| ...
└───packages
    └───x86_64-linux
        ├───default: package
...

# nix build path:./patchelf-baked

Note: this introduces a new builtin derivation (builtin:substitute) with its own new derivation type (DerivationType::Substituted). This derivation is never actually built (i.e. it's a "fake" derivation) - it just pulls in output paths via substitution. It looks like this:

$ nix derivation show path:./patchelf-baked
{
  "derivations": {
    "4aygrnsk0wj7hbalp7rdx078m0y6mbr7-patchelf-0.18.0.drv": {
      "args": [],
      "builder": "builtin:substitute",
      "env": {},
      "inputs": {
        "drvs": {},
        "srcs": []
      },
      "name": "patchelf-0.18.0",
      "outputs": {
        "out": {
          "path": "ingcvxr7h0zwgw1f0vyg41yr9xwi034j-patchelf-0.18.0"
        }
      },
      "system": "builtin",
      "version": 4
    }
  },
  "version": 4
}

The main reason for this is to provide a derivation that other derivations can depend on (e.g. a flake can depend on a baked flake and use its outputs as derivation inputs normally). This is inspired by #408.

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