Use amd64 platform for Molecule tests

[alessfg]

Proposed changes

Explicitly specify amd64 as the platform used in Molecule tests. This will ensure that tests work as expected when run on different host architectures (e.g. newer Macbooks with arm processors).

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)