**What this PR does / why we need it**:

https://github.com/grafana/loki/pull/8081 introduced a bug that caused
the `provisioner` job to create an incorrect secret for the
self-monitoring tenant used by the canary. This fixes that bug, as well
as adding a readme on how to use the `helm-cluster` k3d env.

**Special notes for your reviewer**:

**Checklist**
- [ ] Reviewed the
[`CONTRIBUTING.md`](https://github.com/grafana/loki/blob/main/CONTRIBUTING.md)
guide (**required**)
- [x] Documentation added
- [ ] Tests updated
- [ ] `CHANGELOG.md` updated
- [ ] Changes that require user attention or interaction to upgrade are
documented in `docs/sources/upgrading/_index.md`