download gpg keys for nginz

[jschaul]

On fresh development environments, nginx cannot be compiled without the gpg signature verification in place. Before, this had to be done manually; now GPG keys are downloaded once the first time.

Only relevant when compiling nginz locally.

Checklist

• [x] The PR Title explains the impact of the change.
• [x] The PR description provides context as to why the change should occur and what the code contributes to that effect. This could also be a link to a JIRA ticket or a Github issue, if there is one.