Generate provenance attestations outside of builder

#### What would you like to be added:

As we move towards signing files, we need to move the generation of provenance attestations to happen outside of our builder (`krel`).

#### Why is this needed:

We are currently generating the SLSA provenance attestations inside of krel. This is not a secure practice as a compromised krel build could falsify the provenance data. It is also a blocker for SLSA3.

/cc @kubernetes/release-engineering

Start by reviewing how krel currently generates SLSA provenance attestations and identify the release workflow boundary where attestations must be produced outside the builder. Define the external generation and signing flow, then verify that the resulting provenance cannot be falsified by a compromised krel build and supports the SLSA3 requirement.

Written by the indexing model from the issue text.