Point the worker image at the published Pyroscope 2.2 rock

The worker image in `worker/charmcraft.yaml` is temporarily pinned to an unpublished build so CI can run the v2 charm against a v2 workload. It needs to go back to `ubuntu/pyroscope` before we merge.

The reason for the temporary pin is that v2 needs `-architecture.storage=v2`, and the 1.18 binary rejects that flag outright, so every worker crash-loops and the integration tests time out.

We're waiting on canonical/oci-factory#1176 to publish `2.2-26.04`. The rock itself already merged in canonical/pyroscope-rock#59.

Once it's published:

- repoint `upstream-source` at the published digest
- put the `# renovate: oci-image tag:` annotation back, it was removed so renovate would leave the temporary pin alone
- drop the TODO and the comment above it

The full integration suite passes on a locally built 2.2.1 rock (47 passed, 6 skipped), so this should be the only change needed.

Wait for canonical/oci-factory#1176 to publish 2.2-26.04, then inspect the temporary worker image pin in worker/charmcraft.yaml. Restore the published digest, reinstate the renovate annotation, and remove the TODO and comment; run the full integration suite to confirm the worker no longer crash-loops.

Written by the indexing model from the issue text.