Add flake build file

This commit adds a flake.nix build file, it can be used for three
things:

Build `headscale` from local or straight from git:

nix build
or
nix build github:juanfont/headscale

Run and Build `headscale` from local or straight from git:

nix run
or
nix run github:juanfont/headscale

Set up a development environment including all our tools,
- linters
- protobuf tooling
- compilers

nix develop

flake.lock

@@ -0,0 +1,42 @@
+{
+  "nodes": {
+    "flake-utils": {
+      "locked": {
+        "lastModified": 1644229661,
+        "narHash": "sha256-1YdnJAsNy69bpcjuoKdOYQX0YxZBiCYZo4Twxerqv7k=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "3cecb5b042f7f209c56ffd8371b2711a290ec797",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1646254136,
+        "narHash": "sha256-8nQx02tTzgYO21BP/dy5BCRopE8OwE8Drsw98j+Qoaw=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "3e072546ea98db00c2364b81491b893673267827",
+        "type": "github"
+      },
+      "original": {
+        "id": "nixpkgs",
+        "ref": "nixos-unstable",
+        "type": "indirect"
+      }
+    },
+    "root": {
+      "inputs": {
+        "flake-utils": "flake-utils",
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}