docs: add installation steps for nix

[TroyKomodo]

Adds installation steps for nix after #2518

likely should have been in that PR, but I forgot.. Sorry!

cc @nbp

maybe cc @doronbehar (upstream nixpkgs maintainer for sccache)

not sure if the upstream can take advantage of this create exporting a flake now, just pinging for awareness.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.06%. Comparing base (782b151) to head (28e2348).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2523   +/-   ##
=======================================
  Coverage   71.06%   71.06%           
=======================================
  Files          64       64           
  Lines       35394    35394           
=======================================
  Hits        25153    25153           
  Misses      10241    10241           

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[TroyKomodo]

I will also note that the lack of overlay for the definition of packages imply that this flake creates an additional instances of Nixpkgs (line 54: import nixpkgs) when using any of the modules, which is a bad practice.

(from previous PR)

I tried this flake on my system and I noticed it still does instantiate a separate version of nixpkgs shown by the flake.lock (i think)

{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-24.11";
    sccache.url = "github:troykomodo/sccache/cxx20-modules";
  };

  outputs = { self, nixpkgs, sccache, ... }:
    let
      system = "x86_64-linux";
      pkgs = import nixpkgs {
        inherit system;
        overlays = [ sccache.overlays.default ];
      };
    in {
      devShells.${system}.default = pkgs.mkShell {
        buildInputs = [ pkgs.sccache ];
      };
    };
}

the flake.lock contains

{
  "nodes": {
    "flake-utils": {
      "inputs": {
        "systems": "systems"
      },
      "locked": {
        "lastModified": 1731533236,
        "narHash": "sha256-l0KFg5HjrsfsO/JpG+r7fRrqm12kzFHyUHqHCVpMMbI=",
        "owner": "numtide",
        "repo": "flake-utils",
        "rev": "11707dc2f618dd54ca8739b309ec4fc024de578b",
        "type": "github"
      },
      "original": {
        "owner": "numtide",
        "repo": "flake-utils",
        "type": "github"
      }
    },
    "nixpkgs": {
      "locked": {
        "lastModified": 1765838191,
        "narHash": "sha256-m5KWt1nOm76ILk/JSCxBM4MfK3rYY7Wq9/TZIIeGnT8=",
        "owner": "NixOS",
        "repo": "nixpkgs",
        "rev": "c6f52ebd45e5925c188d1a20119978aa4ffd5ef6",
        "type": "github"
      },
      "original": {
        "owner": "NixOS",
        "ref": "nixos-25.11",
        "repo": "nixpkgs",
        "type": "github"
      }
    },
    "nixpkgs_2": {
      "locked": {
        "lastModified": 1765838191,
        "narHash": "sha256-m5KWt1nOm76ILk/JSCxBM4MfK3rYY7Wq9/TZIIeGnT8=",
        "owner": "NixOS",
        "repo": "nixpkgs",
        "rev": "c6f52ebd45e5925c188d1a20119978aa4ffd5ef6",
        "type": "github"
      },
      "original": {
        "owner": "NixOS",
        "ref": "nixos-25.11",
        "repo": "nixpkgs",
        "type": "github"
      }
    },
    "root": {
      "inputs": {
        "nixpkgs": "nixpkgs",
        "sccache": "sccache"
      }
    },
    "sccache": {
      "inputs": {
        "flake-utils": "flake-utils",
        "nixpkgs": "nixpkgs_2"
      },
      "locked": {
        "lastModified": 1766157939,
        "narHash": "sha256-n0N62klMafQFR1MvktplfR1RZMYX5D5V8T0M3FkiPDM=",
        "owner": "troykomodo",
        "repo": "sccache",
        "rev": "71e9190be7ca9c294be12f764b6f7205b3073780",
        "type": "github"
      },
      "original": {
        "owner": "troykomodo",
        "ref": "troy/cxx20-modules",
        "repo": "sccache",
        "type": "github"
      }
    },
    "systems": {
      "locked": {
        "lastModified": 1681028828,
        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
        "owner": "nix-systems",
        "repo": "default",
        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
        "type": "github"
      },
      "original": {
        "owner": "nix-systems",
        "repo": "default",
        "type": "github"
      }
    }
  },
  "root": "root",
  "version": 7
}

Adding this to the inputs caused the nixpkgs to only be represented once.

sccache.inputs.nixpkgs.follows = "nixpkgs";

Only question is did mess up the overlay or is this expected and its actually fine?