feat(flake): generate nix.flake instead of Docker

...files if a `flake` key exists in a `build.yaml` file.

Refs: OD-37

templates/common/flake.nix.twig

@@ -0,0 +1,23 @@
+{
+  inputs = {
+    devshell.url = "github:numtide/devshell";
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+  };
+
+  outputs = inputs@{ flake-parts, ... }:
+    flake-parts.lib.mkFlake { inherit inputs; } {
+      imports = [ inputs.devshell.flakeModule ];
+
+      systems = [ "x86_64-linux" ];
+
+      perSystem = { config, self', inputs', pkgs, system, ... }: {
+        devshells.default = {
+          packages = with pkgs; [
+{% for package in flake.devshell.packages %}
+            "{{ package }}"
+{% endfor %}
+          ];
+        };
+      };
+    };
+}