Remove unwanted lines from version manifest on tag pipelines

What does this MR do?

It seems omnibus does a git describe --exact-match or git describe --tags while building the manifest file. For non-tag pipelines, this errors out and the error goes to STDERR. However, on tag pipelines, this succeeds and goes to STDOUT. Since we pipe the STDOUT output the version-manifest.json file, the JSON file ends up being invalid JSON for tag pipelines, and trips dependency scanner, as seen in https://dev.gitlab.org/gitlab/omnibus-gitlab/-/jobs/9057452.

This MR is a quick fix dirty hack to remove those extra lines.

Related issues

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
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

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 the GitLab Chart opened