this makes ko run on snapshot builds, too.
the image will be `goreleaser.ko.local:[your tags]`, not sure if we can
change this, seems like we can't.
also fixed a small doc error around it, as well as added a new test to
cover this.
closes#4683
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Signed-off-by: Carlos Alexandro Becker <caarlos0@users.noreply.github.com>
708cd8904d