Make adding annotations and labels easier

[inteon]

In #246, we added the options to add annotations and labels on ko images.
However, the options are hard to use and read due to comma-separated values.
Instead, providing the flag multiple times is easier and allows for cleaner makefiles.

Before:

oci_image_image_label := "org.opencontainers.image.vendor"="Vendor","org.opencontainers.image.authors"="Author","org.opencontainers.image.title"="Title"

After:

oci_image_build_args := \
	--image-label="org.opencontainers.image.vendor"="Vendor" \
	--image-label="org.opencontainers.image.authors"="Author" \
	--image-label="org.opencontainers.image.title"="Title"

[maelvls]

Haven't tested the changes as I didn't know how to that. I'll trust you in that you have tested it with some project 😇

/lgtm
/approve

[cert-manager-prow]

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