Feature request: dry-run mode

`github-action-jira-fixversion` will fail a build when it cannot update the referenced issue. Which is fine. However, if I want to check whether there's a valid issue ref as a pre-merge check, I am currently limited to just searching for something _looking like_ an issue ref (e.g. using github-action-glix), but this isn't good enough, because the issue pointed to may not actually exist.