Use local role name in Molecule tests

[alessfg]

Proposed changes

Use the local role name (ansible-role-nginx) instead of the fully qualified role name (nginxinc.nginx) in Molecule to ensure tests always work as intended in environments where the role has been already installed beforehand.

Checklist

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

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