shell.nix: use tailscale-go for compilation

This change builds a derivation for tailscale-go and makes it available in the
users development environment. This is consistent with the shell.nix in corp/.

Once go1.18 is in a stable Nixpkgs release we can avoid relying on derivations
from nixpkgs head. For now, this works well, and the fetched derivations are
cached in the Nix store according to the usual rules.

Fixes #4231

Signed-off-by: Tom DNetto <tom@tailscale.com>

shell.nix

@@ -1,3 +1,4 @@
+# TODO(tom): Use system nixpkgs exclusively once 1.18 is in a stable release.
 # This is a shell.nix file used to describe the environment that tailscale needs
 # for development. This includes a lot of the basic tools that you need in order
 # to get started. We hope this file will be useful for users of Nix on macOS or
@@ -9,8 +10,32 @@
 # Also look into direnv: https://direnv.net/, this can make it so that you can
 # automatically get your environment set up when you change folders into the
 # project.
-{ pkgs ? import <nixpkgs> {} }:
 
+{
+	pkgs ? import <nixpkgs> {},
+	nixosUnstable ? import (fetchTarball https://github.com/NixOS/nixpkgs/archive/refs/heads/nixpkgs-unstable.tar.gz) { },
+	tailscale-go-rev ? "5ce3ec4d89c72f2a2b6f6f5089c950d7a6a33530",
+	tailscale-go-sha ? "sha256-KMOfzmikh30vEkViEkWUsOHczUifSTiRL6rhKQpHCRI=",
+}:
+let
+	tailscale-go = pkgs.lib.overrideDerivation nixosUnstable.go_1_18 (attrs: rec {
+		name = "tailscale-go-${version}";
+		version = tailscale-go-rev;
+		src = pkgs.fetchFromGitHub {
+		  owner = "tailscale";
+		  repo = "go";
+		  rev = tailscale-go-rev;
+		  sha256 = tailscale-go-sha;
+		};
+		nativeBuildInputs = attrs.nativeBuildInputs ++ [ pkgs.git ];
+		# Remove dependency on xcbuild as that causes iOS/macOS builds to fail.
+		propagatedBuildInputs = [];
+		checkPhase = "";
+		# Our forked tailscale reads this env var to embed the git hash
+		# into the Go build version.
+		TAILSCALE_TOOLCHAIN_REV = tailscale-go-rev;
+	});
+in
 	pkgs.mkShell {
 	  # This specifies the tools that are needed for people to get started with
 	  # development. These tools include:
@@ -18,7 +43,9 @@ pkgs.mkShell {
 	  #  - gotools for goimports, a robust formatting tool for Go source code
 	  #  - gopls, the language server for Go to increase editor integration
 	  #  - git, the version control program (used in some scripts)
-  buildInputs = with pkgs; [
-    go gotools gopls git
+	  buildInputs = [
+	    pkgs.git
+	    nixosUnstable.gotools nixosUnstable.gopls
+	    tailscale-go
 	  ];
 	}