Allow danger to fail on forks

What does this MR do?

Related issues

This change updates the CI pipeline to gracefully fail when there's a fork pipeline that doesn't have $DANGER_GITLAB_API_TOKEN configured. This is helpful when a Community contribution may not have this variable set - it allows the pipeline to continue. Typically, a team member would run a 'merged result' pipeline before merging, anyway. This would give danger the opportunity to run (without failure).

This is a follow-up from !3079 (comment 1357587054)

Resolves #4544 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.