fix: use tipset timestamp in export path name

[lemmih]

Summary of changes

Changes introduced in this pull request:

• Snapshot filenames should use the timestamp of the target epoch instead of now().

Exporting a snapshot at genesis before this PR:
forest_snapshot_calibnet_2023-10-23_height_0.forest.car.zst

After this PR:
forest_snapshot_calibnet_2022-11-01_height_0.forest.car.zst

Reference issue to close (if applicable)

Closes

Other information and links

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation,
• I have added tests that prove my fix is effective or that my feature works (if possible),
• I have made sure the CHANGELOG is up-to-date. All user-facing changes should be reflected in this document.