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>

shell.nix

@@ -7,7 +7,6 @@
 # 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.
-
 (import (
   let
     lock = builtins.fromJSON (builtins.readFile ./flake.lock);
@@ -17,3 +16,4 @@
 ) {
   src =  ./.;
 }).shellNix
+# nix-direnv cache busting line: sha256-imidcDJGVor43PqdTX7Js4/tjQ0JA2E1GdjuyLiPDHI= sha256-+5icFKDHXt3JMbUjLQGes4R+GeUi48xRgGd0yPKVrw0=