Fix NGINX modules not always building with the right tag

What does this MR do?

If the Omnibus build cache were present, previously the ngx_security_headers and nginx-module-vts components would often be built with master instead of tagged versions. This happened because Omnibus' GitFetcher clones the repositories and only checks out the specific version when building the component. The nginx component assumes that the Omnibus source cache has the right version, but if nothing changed with ngx_security_headers and nginx-module-vts then the git checkout will not occur.

To avoid this trouble with GitFetcher, we need to do several things:

1. Copy the extracted source into a subdirectory into /opt/gitlab/src/nginx_modules.

2. Point NGINX to these installed directories.

3. Exclude the /opt/gitlab/src from the final package.

The Omnibus build cache will cache the contents of /opt/gitlab/src in Git, and so the right versions should always be used.

Relates to #9066 (closed)

Changelog: fixed

Related issues

Checklist

See Definition of done.

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Required

• MR title and description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• Latest Merge Result pipeline is green.
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For GitLab team members

If you don't have access to this, the reviewer should trigger these jobs for you during the review process.

• The manual Trigger:ee-package jobs have a green pipeline running against latest commit.
• If config/software or config/patches directories are changed, make sure the build-package-on-all-os job within the Trigger:ee-package downstream pipeline succeeded.
• If you are changing anything SSL related, then the Trigger:package:fips manual job within the Trigger:ee-package downstream pipeline must succeed.
• If CI configuration is changed, the branch must be pushed to dev.gitlab.org to confirm regular branch builds aren't broken.

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.
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