Add script to automate Nix vendorHash updates (#235)

* Add script to automate Nix vendorHash updates

- Create update-nix-hash.sh to automate vendorHash calculation
- Update vendorHash (to be usedafter recent Go dependency changes)
- Script uses ed for OS agnostic in-place editing and extracts hash from nix output

Co-Authored-By: Claude <noreply@anthropic.com>

* Fix Test Nix Flake CI job

* Revert CI change - bd outputs help by default without 'help' arg

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Steve Yegge <stevey@sourcegraph.com>

default.nix

@@ -9,7 +9,7 @@ pkgs.buildGoModule {
   subPackages = [ "cmd/bd" ];
   doCheck = false;
   # Go module dependencies hash (computed via nix build)
-  vendorHash = "sha256-m/2e3OxM4Ci4KoyH+leEt09C/CpD9SRrwPd39/cZQ9E=";
+  vendorHash = "sha256-eUwVXAe9d/e3OWEav61W8lI0bf/IIQYUol8QUiQiBbo=";
 
   # Git is required for tests
   nativeBuildInputs = [ pkgs.git ];