Support uppercase repository names with cosign.

My previous PR didn't properly handle uppercase usernames (or repository names) when signing container images with `cosign`.

It seems that the `docker buildx --push` doesn't like this either, but it's passed the output of the `docker/metadata-action` which seems to lowercase things.

Fixes: #1293

Signed-off-by: Matt Moore <mattmoor@chainguard.dev>

ci/docker-publish.yml

@@ -90,4 +90,4 @@ jobs:
           COSIGN_EXPERIMENTAL: "true"
         # This step uses the identity token to provision an ephemeral certificate
         # against the sigstore community Fulcio instance.
-        run: cosign sign ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}@${{ steps.build-and-push.outputs.digest }}
+        run: cosign sign ${{ steps.meta.outputs.tags }}@${{ steps.build-and-push.outputs.digest }}