[Rolv-Apneseth]

justfile: forward build settings to nested recipes

`just` cmdline overrides apply to a single invocation, with only
environment variables propagating to nested invocations.

test-tmt, for example, calls nested recipes for _build-upgrade-image and
test-tmt-nobuild, so both ran with the default settings. `just
test-composefs systemd ext4 uki sealed` then built the main image as a
sealed UKI but the upgrade image as bls and unsealed (default). The
upgrade image kept a stale UKI whose embedded composefs digest no longer
matched its rootfs, and upgrades to it failed. CI never hit this because
it exports the BOOTC_* env vars, which are propagated.

Let's just forward settings explicitly when we have nested recipes to
avoid this issue. Achieved with a new variable holding the current build
settings as cmdline overrides, which is then passed to every recipe
which invokes another. Recipes which need to override a setting (e.g.
variant=composefs) just pass it after this variable and the last
override wins.

justfile: fix tmt-vm-cleanup to use bcvk libvirt rm-all

`bcvk libvirt rm` takes a single domain name and has no --label option.
The recipe passed --label, so it was failing (on
bcvk-0.21.0-1.fc44.x86_64 at least).

Use `rm-all` which accepts --label and removes every matching domain.