fix fix-cross-device-link error during export

[LesnyRumcajs]

Summary of changes

I encountered an error on my system when trying to export a snapshot.

❯ target/release/forest-cli --chain calibnet snapshot export --tipset 320480
Error: {"code":0,"message":"failed to persist temporary file: Invalid cross-device link (os error 18)"}

The issue is that persist method fails when the temp files live on a different filesystem.

    /// Note: Temporary files cannot be persisted across filesystems. Also
    /// neither the file contents nor the containing directory are
    /// synchronized, so the update may not yet have reached the disk when
    /// `persist` returns.

in my case

/dev/nvme0n1p3  953G  204G  743G  22% /
tmpfs            16G   20M   16G   1% /tmp

Changes introduced in this pull request:

• fallback to copy in case it's impossible to persist the file (which is the case if the temporary file lives on a different filesystem)

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.