Use CI source project for path to exposed job artifacts

What does this MR do?

This fixes the generated path for exposed CI job artifacts when the artifacts are generated by a cross-project MR. In that case, the artifact path would be generated in the destination project's namespace, despite the job having run in the context of the source project.

!23738 (closed) was an attempt to fix this, but it wasn't viable.

Fixes: #37341 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

/cc @engwan