Add option to suppress job summary

[Pesa]

I'd like to disable the built-in job summary and instead generate my own attestation summary and combine it with the summary that I'm already generating for the job. In order to do this, I'd need to:

• pass a flag (or env variable) to actions/attest-build-provenance to suppress the generation of the built-in job summary
• obtain the attestation URL as an output of actions/attest-build-provenance (AFAICS the action doesn't currently provide this information)