Unpin public benchmarks in PRs, make new pinned benchmarks for baselines #12

Per design meeting discussion, we'll:

• Let PRs run on unpinned repos. PRs don't use baselines from the blob store but instead test before/after so that is good
• Copy the benchmarks and pin them, for baselines.
• When we want to upgrade, make a new copy and update it, and eventually remove the old one.
• Split the presets into those for baselines and those for PRs. Likely, one baseline preset only, then a bunch for PRs.