chore: reorder Windows tests and improve their descriptions

[lemeurherve]

This PR puts the slow image build test after the quicker ones (for quicker feedback), uses IMAGE_TAG instead of IMAGE_NAME in the test descriptions (as only the tag/jdk changes), and improve the description of the test building an image with custom build args.

Before:

Describing [jenkins/ssh-agent:nanoserver-ltsc2022-jdk21] image is present
<...>
Describing [jenkins/ssh-agent:nanoserver-ltsc2022-jdk21] build args

After:

Describing [nanoserver-ltsc2022-jdk21] image can be built
Describing [nanoserver-ltsc2022-jdk21] image can be built with custom build args

Extracted from:

• feat: add Windows 2025 images for docker-ssh-agent #581

Testing done

CI

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed