Restrict permissions in `gleam new` generated github actions workflow

It seems worth it to add some hardening/improvements to the GitHub action generated with gleam new, such as setting minimal permissions, disabling persist credentials when checking out, using action version hashes rather than tags, and setting concurrency settings (although this is just a cost/time saver). Presumably the action is included in gleam new as it's intended for people to use it as a jumping off point, and so it feels pertinent that it uses the latest security and performance settings to give people the best/safest experience.