Introduce flake.nix for project

Create flake.nix for the project so that we can reproducibly build this
project using Nix software and on NixOS machines.

flake.nix

@@ -0,0 +1,58 @@
+# This flake was initially generated by fh, the CLI for FlakeHub (version 0.1.12)
+{
+  # Flake inputs
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-24.05";
+    naersk.url = "github:nix-community/naersk";
+    rust-overlay.url = "github:oxalica/rust-overlay";
+  };
+
+  # Flake outputs that other flakes can use
+  outputs = {
+    self,
+    nixpkgs,
+    naersk,
+    rust-overlay,
+  }: let
+    supportedSystems = ["x86_64-linux" "aarch64-linux"];
+    forEachSupportedSystem = f:
+      nixpkgs.lib.genAttrs supportedSystems (system:
+        f rec {
+          pkgs = import nixpkgs {
+            inherit system;
+            overlays = [rust-overlay.overlays.default];
+          };
+          naersk' = pkgs.callPackage naersk {};
+        });
+  in {
+    packages = forEachSupportedSystem ({
+      pkgs,
+      naersk',
+    }: {
+      default = naersk'.buildPackage {
+        src = ./.;
+        nativeBuildInputs = with pkgs; [
+          perl
+          jq
+          openconnect
+          libsoup
+          webkitgtk
+          pkg-config
+          tree
+        ];
+
+        overrideMain = {...}: {
+          postPatch  = ''
+            substituteInPlace crates/gpapi/src/lib.rs \
+              --replace-fail /usr/bin/gpclient $out/bin/gpclient \
+              --replace-fail /usr/bin/gpservice $out/bin/gpservice \
+              --replace-fail /usr/bin/gpgui-helper $out/bin/gpgui-helper \
+              --replace-fail /usr/bin/gpgui $out/bin/gpgui \
+              --replace-fail /usr/bin/gpauth $out/bin/gpauth
+          '';
+        };
+
+      };
+    });
+  };
+}