make sure we have the right json version in the flake

flake.lock

@@ -34,10 +34,28 @@
         "type": "github"
       }
     },
+    "nlohmann-json": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1701207391,
+        "narHash": "sha256-7F0Jon+1oWL7uqet5i1IgHX0fUw/+z0QwEcA3zs5xHg=",
+        "owner": "nlohmann",
+        "repo": "json",
+        "rev": "9cca280a4d0ccf0c08f47a99aa71d1b0e52f8d03",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nlohmann",
+        "ref": "v3.11.3",
+        "repo": "json",
+        "type": "github"
+      }
+    },
     "root": {
       "inputs": {
         "flake-utils": "flake-utils",
-        "nixpkgs": "nixpkgs"
+        "nixpkgs": "nixpkgs",
+        "nlohmann-json": "nlohmann-json"
       }
     },
     "systems": {

flake.nix

@@ -6,9 +6,13 @@
   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs/release-23.11";
     flake-utils.url = "github:numtide/flake-utils";
+    nlohmann-json = {
+      url = "github:nlohmann/json/v3.11.3"; # TODO: read version from file
+      flake = false;
+    };
   };
 
-  outputs = { self, nixpkgs, flake-utils }:
+  outputs = { self, nixpkgs, flake-utils, nlohmann-json }:
     flake-utils.lib.eachDefaultSystem (system:
     let
       pkgs = import nixpkgs { inherit system; };
@@ -61,9 +65,8 @@
           "-DTOMATO_ASAN=OFF"
           "-DCMAKE_BUILD_TYPE=RelWithDebInfo"
 
-          "-DFETCHCONTENT_SOURCE_DIR_JSON=${pkgs.nlohmann_json.src}" # we care less about version here
-          # do we really care less about the version? do we need a stable abi?
-          "-DFETCHCONTENT_SOURCE_DIR_ZSTD=${pkgs.zstd.src}"
+          "-DFETCHCONTENT_SOURCE_DIR_JSON=${nlohmann-json}" # we care about the version
+          "-DFETCHCONTENT_SOURCE_DIR_ZSTD=${pkgs.zstd.src}" # we dont care about the version (we use 1.4.x features)
         ];
 
         # TODO: replace with install command