fix(git-commit-signing): disable curl stderr output

[phorcys420]

The module currently logs some output to stderr when there's no error, this PR fixes that.
I tested it and it works

module "git-commit-signing" {
  source   = "git::https://github.com/phorcys420/coder-modules.git//git-commit-signing?ref=git-commit-signing-fix"
  agent_id = coder_agent.dev.id
}