fix: use valid format to put in github output

[aneojgurhem]

Motivation

Make sure the pipeline using Docker Scout to check the CVEs of our images is working.

Description

• Matrix provided by another job whould be defined with an = not a :.
• Use the right field from the matrix to specify the image.

Testing

Workflow can also be run manually. Il was tested like so.

Impact

No impact.

Checklist

• My code adheres to the coding and style guidelines of the project.
• I have performed a self-review of my code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• I have thoroughly tested my modifications and added tests when necessary.
• Tests pass locally and in the CI.
• I have assessed the performance impact of my modifications.