ci: Add default.nix with exposed pkgs

Allows reusing it in more places

(cherry picked from commit 1700d00588)

ci/default.nix

@@ -0,0 +1,27 @@
+let
+  pinnedNixpkgs = builtins.fromJSON (builtins.readFile ./pinned-nixpkgs.json);
+in
+{
+  system ? builtins.currentSystem,
+
+  nixpkgs ? null,
+}:
+let
+  nixpkgs' =
+    if nixpkgs == null then
+      fetchTarball {
+        url = "https://github.com/NixOS/nixpkgs/archive/${pinnedNixpkgs.rev}.tar.gz";
+        sha256 = pinnedNixpkgs.sha256;
+      }
+    else
+      nixpkgs;
+
+  pkgs = import nixpkgs' {
+    inherit system;
+    config = { };
+    overlays = [ ];
+  };
+in
+{
+  inherit pkgs;
+}

shell.nix

@@ -11,23 +11,12 @@
 #
 #     nix-shell --arg nixpkgs ./.
 #
-let
-  pinnedNixpkgs = builtins.fromJSON (builtins.readFile ci/pinned-nixpkgs.json);
-in
 {
   system ? builtins.currentSystem,
-
+  nixpkgs ? null,
-  nixpkgs ? fetchTarball {
-    url = "https://github.com/NixOS/nixpkgs/archive/${pinnedNixpkgs.rev}.tar.gz";
-    sha256 = pinnedNixpkgs.sha256;
-  },
 }:
 let
-  pkgs = import nixpkgs {
+  inherit (import ./ci { inherit nixpkgs system; }) pkgs;
-    inherit system;
-    config = {};
-    overlays = [];
-  };
 in
 pkgs.mkShellNoCC {
   packages = [