Output SBOM files in separate output group

[mortenmj]

This outputs the created SBOM files from apko_image(). A separate output group is used to avoid getting these intermingled with the files created for the image itself. The SBOM files can be accessed by setting --output_groups=sbomor in a BUILD file:

filegroup(
    name = "foo",
    srcs = [":my_image"],
    output_group = "sbom",
)