circleci: run platformtests against well-defined ZFS releases (#921)

Before this PR we platformtests in CI only against the ZFS version that
shipped with the CircleCI machine image (ZFS 2.2).

Changes in this PR:
- add an `command` for building OpenZFS `.deb`'s from a Git checkout,
and for caching those `.deb`s in CircleCI
- use that command in the platform test build matrix. ZFS versions:
**2.2.9**, **2.3.5**, and **2.4.0**

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -171,6 +171,8 @@ Run `make lint` and `make vet`.
 Update the CI configuration `.circleci/config.yml`:
 - Update Go version references (we reference the minimum and max supported version)
 - Set `Makefile` `RELEASE_GOVERSION` to the new Go version
+- Update the pinned ZFS release tags in the `platformtest` matrix (`zfs_release` parameter) to the latest patch releases of each OpenZFS branch (currently 2.2, 2.3, 2.4)
+- Update the pinned Ubuntu machine image for `platformtest` (e.g. `ubuntu-2404:2025.09.1`) — this is pinned rather than `current` so the ZFS build cache stays valid across runs
 
 Update docs build tooling:
 - Update `uv` version in `.circleci/config.yml` (search for `astral.sh/uv/` and cache keys containing the version)