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 and
`xz2` requirements we no longer need, and add `tar` as a workspace dependency
for the extractor.
Tamir Duberstein