ci: Revamp bootc integration test suite with bcvk VM support

ci: Revamp bootc integration test suite with bcvk VM support

 the old privtest CI job with a new tests/bootc-integration
Rust crate that runs inside a bcvk VM. The old tests/inst crate is
preserved for tests not yet ported.

Tests are split into two tiers based on what they need:

- booted_test!: needs a fully deployed ostree system. Dispatches via
  `bcvk libvirt run` which does `bootc install to-disk`.
- privileged_test!: just needs root. Dispatches via the faster
  `bcvk ephemeral run-ssh`.

The Justfile provides `integration-container` (full suite) and
`integration-ephemeral` (fast privileged-only path). JUnit XML output
is supported via the JUNIT_OUTPUT env var using quick-junit;
integration-container captures results to target/integration-results.xml.

Drop the vendored bootc-ubuntu-setup action in favor of the upstream
bootc-dev/actions/bootc-ubuntu-setup@main.

Assisted-by: OpenCode (Claude claude-opus-4-6)
Signed-off-by: Colin Walters <walters@verbum.org>