crane: Add timestamp to flatten layer

[nivbend]

When flattening an image the blob's created field is the same as the original image's, while the flattened layer's created field is a zero timestamp. This makes it impossible to figure out when was the flattened image actually created and distinguish it from the original.

Set both fields to the current time.

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[nivbend]

Fixes #1996.

[github-actions]

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[nivbend]

@imjasonh @thesayyn @jonjohnsonjr Would you have some time to review this minor fix? Much appreciated.