Bundle handling of Debian kernel archives into xtask so callers can pipe
the raw `.deb` paths straight into `cargo xtask integration-test vm …`.
The driver now extracts each archive into `<cache>/kernel-archives`,
locates the matching `vmlinuz-*`, `lib/modules/*`, and config files, and
feeds those into the initramfs build without requiring the user to
pre-run dpkg/tar. With this in place we drop
`.github/scripts/find_kernels.py`, simplify AGENTS.md/CI instructions to
use `find test/.tmp -name '*.deb'`, remove the gnu-tar requirement we no
longer need, and add `tar` as a workspace dependency for the extractor.
Tamir Duberstein