Add best-effort flake.nix

The purpose for this flake is to make `nix run nix/2.3-maintenance` work,
so that it's easier to answer questions about this old series of releases.

flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1705133751,
+        "narHash": "sha256-rCIsyE80jgiOU78gCWN3A0wE0tR2GI5nH6MlS+HaaSQ=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "9b19f5e77dd906cb52dade0b7bd280339d2a1f3d",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake.nix

@@ -0,0 +1,56 @@
+{
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+  # A very basic flake to conveniently `nix run nix/2.3` for testing.
+  # Note that this release has no built-in support for flakes,
+  # and this flake declaration is provided on a best-effort basis.
+  outputs = { self, nixpkgs, ... }:
+    let
+      inherit (nixpkgs) lib;
+      inherit (lib) flip genAttrs mapAttrs substring;
+      rev = self.sourceInfo.rev or self.sourceInfo.dirtyRev or "";
+      revCount = self.sourceInfo.revCount or 0;
+      shortRev = self.sourceInfo.shortRev or (substring 0 7 rev);
+      release = import ./release.nix {
+        nix = {
+          outPath = ./.;
+          inherit rev revCount shortRev;
+        };
+        nixpkgs = nixpkgs.outPath;
+        officialRelease = false;
+      };
+    in
+  {
+    # inherit release;
+    packages =
+      mapAttrs
+        (system: nix: {
+          default = nix;
+          nix = nix;
+        })
+        release.build;
+    apps =
+      mapAttrs
+        (system: packages:
+          let
+            appFor = mainProgram: {
+              type = "app";
+              program = lib.getExe' packages.nix mainProgram;
+            };
+          in flip genAttrs appFor [
+            "nix"
+            "nix-build"
+            "nix-channel"
+            "nix-collect-garbage"
+            "nix-copy-closure"
+            "nix-daemon"
+            "nix-env"
+            "nix-hash"
+            "nix-instantiate"
+            "nix-prefetch-url"
+            "nix-shell"
+            "nix-store"
+          ])
+        self.packages;
+  };
+}
+