Abort if no jq is found
I had the docs:clean_redirects fail part way through the process because I didn't have jq installed.
This MR ensures jq is available before trying to do anything. jq isn't required until: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/4172fb5ed0aa65091c02ec9cd54cacb8532ba97b/lib/task_helpers.rb#L83, but lots of project checkouts are affected before it fails if we wait until then to check.