ci: Use default signature filename for cosign

Author: armsnyder

.github/.goreleaser.yaml

@@ -49,7 +49,6 @@ notarize:
 binary_signs:
   - cmd: cosign
     stdin: "{{ .Env.COSIGN_PWD }}"
-    signature: "${artifact}.sig"
     args:
       - sign-blob
       - --key=env://COSIGN_KEY