fix: populate OCI platform metadata & do not ignore platform option for...

[jasonpaulos]

…single-platform images

This PR makes two improvements when reading OCI directory and tarball images:

1. Populates the image metadata struct with the appropriate platform information. The only case when this is omitted is when no specific platform was requested and the OCI image contains only a single image.
2. When an OCI image contains only a single image, the prior behavior would be to select this image regardless of the requested platform. Now, an error will be thrown if a platform was requested that does not match the single image. If the user does not request a specific platform, the single image will continue to be used regardless of its platform.

I view number 2 as a bugfix, since silently returning an image for a different platform than the one requested is not expected behavior. Additionally, a proper error is already returned if the desired platform isn't available on an OCI bundle that has more than 1 image in it.

[jasonpaulos]

@wagoodman I just added --retry-times=3 to the skopeo copy command, which should help with the integration test that just flaked.