update-flake.sh: tooling to keep Nix SRI hashes in sync.

Also fixes the Go toolchain SRI hash from a7f05c6bb0,
it turns out I initialized the file with an SRI hash for an older
toolchain version, and because of the unique way fixed-output derivations
work in nix, nix didn't tell me about the mismatch because it just
cache-hit on the older toolchain and moved on. Sigh.

Updates #6845.

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

flake.nix

@@ -141,14 +141,17 @@
       };
       devShell = pkgs.mkShell {
         packages = with upstreamPkgs; [
-          pkgs.tailscale_go
+          curl
           git
-          gotools
           gopls
+          gotools
           graphviz
+          perl
+          pkgs.tailscale_go
         ];
       };
     };
   in
     flake-utils.lib.eachDefaultSystem (system: flakeForSystem nixpkgs system);
 }
+# nix-direnv cache busting line: sha256-imidcDJGVor43PqdTX7Js4/tjQ0JA2E1GdjuyLiPDHI= sha256-+5icFKDHXt3JMbUjLQGes4R+GeUi48xRgGd0yPKVrw0=