ci: fix env var interpolation

[peterdeme]

Description of the change

Unfortunately it's a bit more complicated to inject those env vars manually so we opted to Packer to just get them from the "basic" env vars.

Type of change

• Bug fix (non-breaking change that fixes an issue);
• New feature (non-breaking change that adds functionality);
• Breaking change (fix or feature that would cause existing functionality to not work as expected);
• Documentation (a documentation or example fix not affecting the infrastructure managed by this module);

Checklists

Development

• All necessary variables have been defined, with defaults if applicable;
• The HCL code is formatted;
• An AMI has been created in some AWS account, and the AMI is working as expected;

Code review

• This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached;
• This pull request is no longer marked as "draft";
• Reviewers have been assigned;
• Changes have been reviewed by at least one other engineer;