Fallback to copying when hardlinking does not work

[rhpvorderman]

Hardlinking works well in the cromwell implementation, but not in the miniwdl implementation. So use copying as a fallback.

Checklist

• [ ] Pull request details were added to CHANGELOG.md.
• [ ] Documentation was updated (if required).
• [ ] parameter_meta was added/updated (if required).
• [ ] Submodule branches are on develop or a tagged commit.