Document workaround for passing environment variables as artifacts

• Documents Feature Artifacts
• Follow-up from: https://gitlab.com/gitlab-org/gitlab-ce/issues/42387#note_133254327

New doc or update?

• New documentation
• Update existing documentation

Checklists

Product Manager

• Add the correct labels
• Add the correct milestone
• Indicate the correct document/directory for this feature
• Fill the doc blurb below

Documentation blurb

This update doesn't seem to have a blurb as described here, please let me know if I've misunderstood something.

• Doc this is not a new document

  This is not a new document

• Feature this is not a new feature

  This is not a new feature

• Feature this does not introduce new use cases

  This does not introduce new use cases

Developer

• Copy the doc blurb above and paste it into the doc
• Write the tutorial - explain how to use the feature
• Submit the MR using the appropriate MR description template