vine: only checkpoint DaskVineFile's

[btovar]

Proposed changes

Please describe your changes (e.g., what problems they attempt to solve, what results are expected, etc.) Additional motivation and context are welcome.
Please also mention relevant issues and pull requests as appropriate.

Post-change actions

Put an 'x' in the boxes that describe post-change actions that you have done.
The more 'x' ticked, the faster your changes are accepted by maintainers.

• make test Run local tests prior to pushing.
• make format Format source code to comply with lint policies. Note that some lint errors can only be resolved manually (e.g., Python)
• make lint Run lint on source code prior to pushing.
• Manual Update Did you update the manual to reflect your changes, if appropriate? This action should be done after your changes are approved but not merged.
• Type Labels Select github labels for the type of this change: bug, enhancement, etc.
• Product Labels Select github labels for the product affected: TaskVine, Makeflow, etc.
• PR RTM Mark your PR as ready to merge.

Additional comments

This section is dedicated to changes that are ambitious or complex and require substantial discussions. Feel free to start the ball rolling.