Fix build under nix restricted eval mode

ci/default.nix

-let sources = import ./nix/sources.nix;
-in
-
-{ nixpkgs ? (import sources.nixpkgs.outPath {}) }:
+{ sources ? import ./nix/sources.nix, nixpkgs ? (import sources.nixpkgs.outPath {}) }:
 
 with nixpkgs;
 let

flake.nix

@@ -11,10 +11,10 @@
   inputs.overlay-tool.flake = false;
   inputs.flake-compat.url = "github:edolstra/flake-compat";
   inputs.flake-compat.flake = false;
-  outputs = {nixpkgs, ...}: 
+  outputs = sources@{nixpkgs, ...}: 
   let 
     system = "x86_64-linux";
-    env = import ./ci/default.nix { nixpkgs = nixpkgs.legacyPackages."${system}"; };
+    env = import ./ci/default.nix { inherit sources; nixpkgs = nixpkgs.legacyPackages."${system}"; };
   in {
     devShells."${system}".default = env;
     hydraJobs.env."${system}" = env;