Fix cosign verification

[AnHeuermann]

Issue

images=""
  for tag in ${TAGS}; do
    images+="${tag}@${DIGEST} "
  done
  cosign verify ${images}

Error: --certificate-identity or --certificate-identity-regexp is required for verification in keyless mode
error during command execution: --certificate-identity or --certificate-identity-regexp is required for verification in keyless mode

Changes

• Make for loop more robust
• Fix cosign verify call.