refactor: replace fetchTarball with flake input for nixpkgs-oldstable

Improves reproducibility and avoids evaluation-time network access by
using a proper flake input instead of fetchTarball for the older nixpkgs
needed by pg_net and plv8.

Author: yvan-sraka

flake.lock

@@ -22,6 +22,7 @@
     nix2container.inputs.nixpkgs.follows = "nixpkgs";
     nix2container.url = "github:nlewo/nix2container";
     nixpkgs-go124.url = "github:Nixos/nixpkgs/d2ac4dfa61fba987a84a0a81555da57ae0b9a2b0";
+    nixpkgs-oldstable.url = "github:NixOS/nixpkgs/a76c4553d7e741e17f289224eda135423de0491d";
     nixpkgs.url = "https://channels.nixos.org/nixos-unstable/nixexprs.tar.xz";
     rust-overlay.inputs.nixpkgs.follows = "nixpkgs";
     rust-overlay.url = "github:oxalica/rust-overlay";

flake.nix

@@ -7,14 +7,11 @@
   libuv,
   makeWrapper,
   switch-ext-version,
+  nixpkgs-oldstable,
 }:
 
 let
-  olderPkgs = import (fetchTarball {
-    url = "https://github.com/nixos/nixpkgs/archive/a76c4553d7e741e17f289224eda135423de0491d.tar.gz";
-    sha256 = "0rwdzp942b8ay625lqgra83qrp64b3wqm6w9a0i4z593df8x822v";
-  }) { system = stdenv.system; };
-  curl = olderPkgs.curl;
+  curl = nixpkgs-oldstable.curl;
 in
 let
   pname = "pg_net";

nix/ext/pg_net.nix

@@ -10,14 +10,11 @@
   clang,
   xcbuild,
   patchelf,
+  nixpkgs-oldstable,
 }:
 let
   # plv8 3.1 requires an older version of v8 (we cannot use nodejs.libv8)
-  node_pkgs = import (fetchTarball {
-    url = "https://github.com/nixos/nixpkgs/archive/a76c4553d7e741e17f289224eda135423de0491d.tar.gz";
-    sha256 = "0rwdzp942b8ay625lqgra83qrp64b3wqm6w9a0i4z593df8x822v";
-  }) { system = stdenv.system; };
-  inherit (node_pkgs) v8;
+  inherit (nixpkgs-oldstable) v8;
 in
 stdenv.mkDerivation (finalAttrs: {
   pname = "plv8";

nix/ext/plv8.nix

@@ -14,6 +14,7 @@
   buildEnv,
   nodejs_20,
   libcxx,
+  nixpkgs-oldstable,
 }:
 
 let
@@ -36,11 +37,7 @@ let
   );
 
   # plv8 3.1 requires an older version of v8 (we cannot use nodejs.libv8)
-  node_pkgs = import (fetchTarball {
-    url = "https://github.com/nixos/nixpkgs/archive/a76c4553d7e741e17f289224eda135423de0491d.tar.gz";
-    sha256 = "0rwdzp942b8ay625lqgra83qrp64b3wqm6w9a0i4z593df8x822v";
-  }) { system = stdenv.system; };
-  inherit (node_pkgs) v8;
+  inherit (nixpkgs-oldstable) v8;
 
   # Build function for individual versions
   build =

nix/ext/plv8/default.nix

@@ -10,6 +10,12 @@
         overlays = [
           (import inputs.rust-overlay)
           self.overlays.default
+          (_final: _prev: {
+            nixpkgs-oldstable = import inputs.nixpkgs-oldstable {
+              inherit system;
+              config.allowUnfree = true;
+            };
+          })
         ];
       };
     };