nix: add nix flake

Since I use NixOS, its only natural to add a flake development.

flake.lock

@@ -0,0 +1,26 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1636267212,
+        "narHash": "sha256-KDS173KqmqrYUPY9N4vf750GxIo+S6E0djyq2BsQm8s=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "c935f5e0add2cf0ae650d072c8357533e21b0c35",
+        "type": "github"
+      },
+      "original": {
+        "id": "nixpkgs",
+        "ref": "nixos-unstable",
+        "type": "indirect"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake.nix

@@ -0,0 +1,21 @@
+{
+  description = "Nix flake for dumb-jose development.";
+
+  inputs = {
+    nixpkgs.url = "nixpkgs/nixos-unstable";
+  };
+
+  outputs = { self, nixpkgs }:
+    let
+      pkgs = nixpkgs.legacyPackages.x86_64-linux;
+    in
+    {
+      devShell.x86_64-linux = pkgs.mkShell {
+        buildInputs = with pkgs; [
+          delve
+          go_1_17
+          gopls
+        ];
+      };
+    };
+}