[docs][RELEASE.md] Script creating slsa-verifier test data

[ianlewis]

Getting the generated test data for new releases can be scripted to download it from the GHA runs and copy them into the right place in the slsa-verifier repo.

For example get latest run for v14 and download the artifacts

RUN_ID=$(gh run list -R slsa-framework/example-package -b v14 -w .github/workflows/verifier-e2e.all.workflow_dispatch.main.all.slsa3.yml --json databaseId | jq '.[] .databaseId' | head -n1)
gh run download -R slsa-framework/example-package "${RUN_ID}"

Right now the steps for adding the files is very manual and takes a lot of time.