Per actions/runner-images#10636, the
`ubuntu-latest` GitHub Actions hosted runner label is being
migrated from aliasing `ubuntu-22.04` to aliasing `ubuntu-24.04`.
Our `ci.yml` workflow, which includes the full `test` job, uses
`ubuntu-latest`, and recently has switched automatically to using
an Ubuntu 24.04 runner.

It makes sense to use this newer version, but that runner
apparently does not preinstall the headers required to build with
`-llzma`. That causes `just ci-test` to fail at
`cargo nextest run -p gix-testtools --features xz` with
`/usr/bin/ld: cannot find -llzma: No such file or directory`.

This commit installs the `liblzma-dev` package that provides the
required headers, so we can use Ubuntu 24.04 LTS while continuing
to test the `xz` feature of `gix-testtools`.

The failure this addresses was first observed in GitoxideLabs#1618 at 64e0f78:
https://github.com/GitoxideLabs/gitoxide/actions/runs/11304216949/job/31442302018?pr=1618

But it is not caused by any of the changes there, and it now occurs
whenever the `test` job is run, including if it is re-run at the
tip of the main branch (or any other branch), such as in:
https://github.com/EliahKagan/gitoxide/actions/runs/11317510844/job/31471040361

In both cases, expanding *Set up job* > *Operating system* shows
Ubuntu 24.04.1 LTS.

(The failure this addresses is also not related to GitoxideLabs#1622, which
documents an unrelated failure that is possible to observe locally
and that, if no change is made related to it, can be expected to
occur on CI in the `test` starting sometime soon, but that is not
yet seen on CI.)