kernel-headers-test: bullseye LTS ended, apt 404s on most boards

[klutchell]

kernel-headers-test fails on most boards because apt can't fetch its build
deps. Debian 11 bullseye LTS ended 2026-08-31, so this isn't a mirror glitch and
it won't fix itself.

The final bullseye-security index went out on the EOL date and expires
2026-09-07 21:13 UTC. Packages are being deleted from the live pool while that
index still advertises them, which is what gives us the 404s:

Package	deb.debian.org	archive.debian.org
linux-libc-dev 5.10.262-1	404	404
libssl-dev 1.1.1w-0+deb11u8	404	404
libc-dev-bin 2.31-13+deb11u14	200	404

archive.debian.org/debian/dists/bullseye/ is up but
archive.debian.org/debian-security/dists/bullseye-security/ still 404s, so the
packages have left the live mirrors and haven't reached the archive. Removal is
propagating unevenly across CDN backends, so the same build can pass on one
runner and fail on the next.

Two deadlines. Valid-Until expires 2026-09-07, after which apt rejects the
whole index instead of individual files. And whenever bullseye-security does
reach archive.debian.org, the pool paths change again.

This takes out 18 of 24 required checks on #3935.