Allow `apko_image.tag` to be unspecified when `output = "oci"`

[plobsing]

The tag parameter to apko_image is documented as

tag to apply to the resulting docker tarball. only applicable when output is docker

Nevertheless, the argument is always required, even when specifying an OCI build.

It seems to me that in the OCI case, providing a value for tag makes for a misleading build file, since any value chosen will not have any effect on the built artifacts. I would prefer to omit the argument in this case.