Basic Nix-based build using nix develop -c gradle jpackage

This is not a full (i.e. isolated, reproducible) Nix build, but does
verify that the Nix devShell in `flake.nix` works and that we can successfully
build Sparrow with Gradle and JDK provided by Nix.

Author: msgilligan

.github/workflows/nix-jpackage.yaml

@@ -0,0 +1,24 @@
+name: Nix Package
+
+on: workflow_dispatch
+
+permissions:
+  contents: read
+
+jobs:
+  nix_jpackage:
+    name: jpackage Nix ${{ matrix.os }}
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-22.04, ubuntu-22.04-arm, macos-13, macos-14]
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          submodules: true
+      - name: Install Nix
+        uses: cachix/install-nix-action@v30
+        with:
+          github_access_token: ${{ secrets.GITHUB_TOKEN }}
+      - name: Build in Nix development shell
+        run: nix develop -c gradle jpackage

flake.lock

@@ -0,0 +1,36 @@
+{
+  description = "Sparrow Wallet Nix Flake (Development Shell only, for now)";
+
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
+
+    flake-parts = {
+      url = "github:hercules-ci/flake-parts";
+      inputs.nixpkgs-lib.follows = "nixpkgs";
+    };
+
+    devshell = {
+      url = "github:numtide/devshell";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+  };
+
+  outputs = inputs @ { flake-parts, devshell, ... }:
+    flake-parts.lib.mkFlake {inherit inputs;} {
+      systems = [ "x86_64-linux" "x86_64-darwin" "aarch64-linux" "aarch64-darwin" ];
+
+      perSystem = { config, self', inputs', pkgs, system, lib, ... }: let
+        inherit (pkgs) stdenv;
+      in {
+        # define default devshell
+        devShells.default = pkgs.mkShell {
+          packages = with pkgs ; [
+                jdk23                # This JDK will be in PATH
+                (gradle.override {   # Gradle 8.x (Nix package) runs using an internally-linked JDK
+                    java = jdk23;    # Run Gradle with this JDK
+                })
+            ];
+        };
+      };
+    };
+}