prevent push with tar layers for SIF with overlay

[dtrudg]

When an OCI-SIF image is push-ed to an OCI registry, and it is requested to use the tar layer format (#2848) then overlays must be handled as part of the push process:

• An extfs overlay must be sealed (singularity overlay seal conversion of overlay to r/o layer in OCI-SIF #2870) into a read-only layer.
• The layer must be pushed in tar format.

[dtrudg]

On second thought, I'm not sure whether we want to do this automatically... or whether we should fail if there's an overlay, with a message that the image needs to be sealed first.

[dtrudg]

We already fail when pushing an image with an overlay with --layer-format=tar

$ singularity push alpine_latest.oci.sif docker://docker.io/dctrud/test-oci-sif
FATAL:   Unable to push image to oci registry: unexpected layer mediaType: application/vnd.sylabs.image.layer.v1.ext3

Could just make it a bit cleaner - check for the overlay up-front and fail early.