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.

[adambkaplan]

/kind feature

[SaschaSchwarze0]

Tekton Chains looks for results, shp-output-image is a param.

[sbose78]

Correct, needs changes to the build strategy to write that out to a result. Got it working locally 🕹️

[SaschaSchwarze0]

Right, maybe we introduce a tekton-chains-enabled toggle in the controller config and if it is one it will take care of it (by adding a post-step to write the two results if possible).

[adambkaplan]

Marking this as needing a SHIP, since this will impact how build strategy implementations.