fix for serial transfer from non-zero rank client

[MichaelBrim]

Motivation and Context

When using the unifyfs-stage utility in serial mode for multi-node jobs (either directly or via the unifyfs utility staging support), and more than one file is present in the manifest file, the file transfers for non-rank 0 clients would fail due to a bug in our logic for creating the destination file.

How Has This Been Tested?

Tested on OLCF Summit.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Performance enhancement (non-breaking change which improves efficiency)
• [ ] Code cleanup (non-breaking change which makes code smaller or more readable)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Testing (addition of new tests or update to current tests)
• [ ] Documentation (a change to man pages or other documentation)

Checklist:

• [x] My code follows the UnifyFS code style requirements.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.
• [x] All commit messages are properly formatted.