nix: add agenix as flake input

flake.nix

@@ -5,9 +5,10 @@
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
     flake-utils.url = "github:numtide/flake-utils";
     rust-overlay.url = "github:oxalica/rust-overlay";
+    agenix.url = "github:ryantm/agenix";
   };
 
-  outputs = { self, nixpkgs, flake-utils, rust-overlay }:
+  outputs = { self, nixpkgs, flake-utils, rust-overlay, agenix }:
     flake-utils.lib.eachDefaultSystem (system:
       let
         pkgs = import nixpkgs {
@@ -17,6 +18,9 @@
         rust = pkgs.rust-bin.stable.latest.default.override {
           targets = [ "wasm32-unknown-unknown" ];
         };
+        agenixPkg =
+          if pkgs ? agenix then pkgs.agenix
+          else agenix.packages.${system}.default;
         webkitgtk =
           if pkgs ? webkitgtk_4_1 then pkgs.webkitgtk_4_1
           else if pkgs ? webkitgtk_4_0 then pkgs.webkitgtk_4_0
@@ -46,7 +50,7 @@
             ffmpeg
             nodejs_22
             nodePackages.wrangler
-            agenix
+            agenixPkg
             forgejo-cli
             uv
             git
@@ -75,7 +79,7 @@
             pkg-config
             openssl
             ffmpeg
-            agenix
+            agenixPkg
             forgejo-cli
             git
           ];