Use CI_PROJECT_DIR-relative paths for GitLab annotations

[charliermarsh]

At least for Gitlab path should be relative to the CI_PROJECT_DIR location. With this implementation links in widgets are broken.

Originally posted by @Agalin in #1837 (comment)

[Agalin]

I'd say there are two options:

• Make it relative to the mentioned variable if present - but what happens if it's not defined?
• Keep paths just relative. I'm not familiar with Github's directory changing but I see no reason why toll cannot be called from a parent directory. It's already possible to use a config from subdirectory and select only subdir files for scan.

[charliermarsh]

I guess my initial reaction is that we should try to make relative to CI_PROJECT_DIR, then fallback to cwd-relative if it's not defined.