Minor changes needed to adhere to tekton-provenance format

[sbose78]

Modify name of literals to "automatically" have Tekton Chains push signatures and attestation. See format.

• Modify shp-output-image to *IMAGE_URL
• write out IMAGE_URL to a result ( update to step generation )
• modify result named shp-image-digest to *IMAGE_DIGEST

Here's how the diff looks like: https://www.diffchecker.com/S6oLSCE0

I was successful upon making the above changes manually.