image list: do not reformat .CreatedAt

As shown in containers#14456, the `.CreatedAt` fields for `image list` and of
`image history` can differ by one.  The discussed theory is that the
off-by-one is caused by rounding.

Indeed, the field of `image list` is reformatted.  `image history` is
returning the UNIX time; just as the `.CreatedAt` field should.

I am unable to create a reproducer for the issue but double-checked
what the docker client does: return the UNIX time.

[NO NEW TESTS NEEDED]

Fixes: containers#14456
Signed-off-by: Valentin Rothberg <[email protected]>

cmd/podman/images/list.go

@@ -172,7 +172,7 @@ func writeJSON(images []imageReporter) error {
 		var h image
 		h.ImageSummary = e.ImageSummary
 		h.Created = e.ImageSummary.Created
-		h.CreatedAt = e.created().Format(time.RFC3339Nano)
+		h.CreatedAt = e.created().String()
 		h.RepoTags = nil
 
 		imgs = append(imgs, h)