docs: improve help text for ci run --variables-from

• Please check this box if this contribution uses AI-generated content (including content generated by GitLab Duo features) as outlined in the GitLab DCO & CLA. As a benefit of being a GitLab Community Contributor, you receive complimentary access to GitLab Duo.

Prior to this change, users could not easily discover the format for the variables JSON file to pass to glab ci run.

Add an explanation to the --help text so that users can learn how to construct this file.

How has this been tested?

compiled and ran ./bin/glab ci run -h

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap