Ensure gpg-agent is installed on Ubuntu

[coaxial]

Proposed changes

Makes sure gpg-agent is present on Ubuntu systems before importing the apt keys, as outlined in #507.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• I have added Molecule tests that prove my fix is effective or that my feature works
• I have checked that any relevant Molecule tests pass after adding my changes
• I have updated any relevant documentation (defaults/main/*.yml, README.md and CHANGELOG.md)

[alessfg]

I would probably add the package requirement here instead https://github.com/nginxinc/ansible-role-nginx/blob/main/vars/main.yml#L65 given it seems to be a hard requirement for Debian based systems.

[coaxial]

Thanks for the feedback, I have made the change.

Does this require a Molecule test added?

[alessfg]

I think it's safe to assume most systems have gpg-agent installed. I don't know why the image created by geerlingguys's role does not? As such, creating a Molecule test for this specific use case might be more effort than it's worth. That being said, can you update the Changelog? I'll merge the PR once that's done.

[coaxial]

Done.