Try to fix devenv issue with substituters

Will need to wait for cachix/devenv#1532 to land
in a new release.

nixos/flake-module.nix

@@ -13,6 +13,15 @@
           };
           options = "--delete-older-than 30d";
         };
+
+        settings = {
+          substituters = [
+            "https://devenv.cachix.org"
+          ];
+          trusted-public-keys = [
+            "devenv.cachix.org-1:w1cLUi8dv3hnoSPGAuibQv+f9TZLr6cv/Hm9XgU50cw="
+          ];
+        };
       };
 
       nixpkgs.config.allowUnfree = true;