xargs is not required to do keyless signing with GH OIDC token

[jbtrystram]

The documentation in the README suggests using xargs to sign containers/blobs without a key.
This is not needed and add complexity to the command.

I was able to sucessfully sign images from a GH action workflow with the following step:

- name: Sign the images with GitHub OIDC Token
      env:
        IMAGE: {{ env.IMAGE_REGISTRY}}/${{ env.REGISTRY_USER}}/${{ env.IMAGE_NAME }}
      run: | 
        cosign sign \
        -a "repo=${{ github.repository }}" \
        -a "workflow=${{ github.workflow }}" \
        -a "ref=${{ github.sha }}" \
        --yes ${IMAGE}@${DIGEST}

I drew inspiration from there : https://github.com/sigstore/cosign/blob/main/scripts/sign-images-ci.sh