crane: Add timestamp to flatten layer

[Stephanie0829]

Fixes #2115, which was reopened from #1974.

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.

After using crane flatten, the new layer does not have created time set. This will set the created time to the current time.

Tested:

./hack/presubmit.sh 

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[Stephanie0829]

Hi! Created this PR for visibility since the previous PR was labeled as stale. @jonjohnsonjr @Subserial @imjasonh @brandonmichigangithub Mind reviewing this so these changes can be merged (much appreciated)?

[Stephanie0829]

@brandonmichigangithub The lint failure is unrelated to my changes. Do you mind merging this PR? Thanks!

[brandonmichigangithub]

Can you edit the lint ignore flag such that you can ignore this failure?

[brandonmichigangithub]

Actually, can you rebase this pull request? This issue should be fixed with #2137

[Stephanie0829]

@brandonmichigangithub Hello! I updated the branch to incorporate the fix but need maintainer approval to run the workflow.

[Stephanie0829]

@brandonmichigangithub Hello! Bumping the previous message.

@brandonmichigangithub Hello! I updated the branch to incorporate the fix but need maintainer approval to run the workflow.