ci: commit lint works for forks without merge trains enabled

Description

This MR fixes the commit-lint job for forks that don't use merge result pipelines and merge trains.

If the script finds out that it runs as a part of a "detached" pipeline, it will change behaviour to change all commits that will be added by the MR.

The script also shows a warning because without merge trains, it can't guarantee that the commits have correct messages.

Related Issues

Related to #820 (closed)

Previous MR !1357 (merged)

How has this been tested?

• I created an MR in the renovate fork and it passes: gitlab-renovate-forks/gitlab-vscode-extension!1 (closed)
• I created an MR in my fork (with merge trains enabled) and I check that the linting still detects invalid commits: https://gitlab.com/viktomas/gitlab-vscode-extension/-/jobs/6070580132

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