flake.nix: use Go 1.21 to build tailscale, for real this time

The previous change just switched the Go version used in the dev
environment (for use with e.g. direnv), not the version used for
the distribution build. Oops.

Updates #cleanup

Signed-off-by: David Anderson <danderson@tailscale.com>

flake.nix

@@ -70,7 +70,7 @@
     # So really, this flake is for tailscale devs to dogfood with, if
     # you're an end user you should be prepared for this flake to not
     # build periodically.
-    tailscale = pkgs: pkgs.buildGo120Module rec {
+    tailscale = pkgs: pkgs.buildGo121Module rec {
       name = "tailscale";
 
       src = ./.;