Set vexctl generate output as action output

[pdxjohnny]

Example usage:

    - uses: openvex/generate-vex@159b7ee4845fb48f1991395ce8501d6263407360
      name: Run vexctl
      id: vexctl
      with:
        product: pkg:github/${{ github.repository }}@${{ github.sha }}
    - name: Submit OpenVEX to SCITT
      id: scitt-submit-openvex
      uses: scitt-community/scitt-api-emulator@f1f5c16630a28511e970b6903fbc4c0db6c07654
      with:
        issuer: did:web:raw.githubusercontent.com:intel:dffml:public-keys:authorized_keys
        subject: pkg:github/${{ github.repository }}@${{ github.sha }}
        payload: ${{ steps.vexctl.outputs.openvex }}
        private-key-pem: private-key.pem
        scitt-url: https://scitt.unstable.chadig.com