podman build --timestamp is a NOP

```console
$ printf "FROM quay.io/libpod/alpine:latest\nRUN true\n" | ./bin/podman build --quiet --timestamp=1 -t foo -
31f377d423d4b50ba9bbaa69c14d81f87c5da62c9509a029e5ec805690fe15e0
$ ./bin/podman images foo
REPOSITORY     TAG     IMAGE ID      CREATED        SIZE
localhost/foo  latest  31f377d423d4  6 seconds ago  5.85 MB
                       ^^^^^^^^^^^^--- expected: 1970
```