Don't sign git tags #283

Rationale: when the git config setting `tag.gpgSign=true` is used, this implies the `-s` option to `git tag`. This means that git needs a message for the tag. Because no message is given, git invokes an editor, which doesn't work when running non-interactively. We don't need signed tags in a git repo created by HaikuPorter, so the easiest solution is to pass `--no-sign` to `git tag` calls.