Add `s390x` as a platform for Molecule tests

[alessfg]

Proposed changes

BUG FIXES:

Remove the arch option from the Debian family NGINX repository source. In its current form this prevented the role from working any platforms beyond x86_64/amd64 and aarch64/arm64.

TESTS:

Explicitly test some distributions using aarch64 and s390x as the Molecule platform. This should ensure the role works as intended across the various architectures that are officially supported.

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)