[Tools] Do not link cta-admin-grpc with objectstore when using pgsched

Description

Previously we were incorrectly linking with the objectstore library, without conditionally checking whether the postgres scheduler is being used. This made it impossible to deploy the postgres scheduler with cta-admin-grpc installed. This commit fixes the problem by checking and only linking with the objectstore if necessary.

Checklist

• Documentation reflects the changes made.
• Merge Request title is clear, concise, and suitable as a changelog entry. See our contributing docs

References

Closes #1339 (closed)