CI: Linux build jobs failing due to 404s on Debian bullseye-security packages

Problem Description

CI build jobs are failing on all Linux builds because the pinned Debian bullseye Docker image's apt sources reference package versions that no longer exist on deb.debian.org's bullseye-security mirror. This looks like a permanent removal (packages superseded by a newer point release), not a transient network blip -- the same failure has recurred across multiple separate CI runs, over several days, on multiple unrelated PRs.

Steps to reproduce the behavior:

1. Open any recent PR that triggers the build (ubuntu-latest, linux-x86_64, ...) or build (ubuntu-24.04-arm, linux-arm64, ...) CI job.
2. Look at the "Install build dependencies" (apt-get) step's log.
3. Multiple packages fail to download with 404 errors from deb.debian.org/debian-security, e.g.:
   • perl-modules-5.32_5.32.1-4+deb11u5
   • libperl5.32_5.32.1-4+deb11u5
   • openssl_1.1.1w-0+deb11u8
   • xz-utils_5.2.5-2.1~deb11u2
   • linux-libc-dev_5.10.262-1
   • libexpat1_2.2.10-2+deb11u7

Example failing run: https://github.com/mitmproxy/mitmproxy/actions/runs/34699885016/job/103569646713

This likely needs the pinned Debian base image bumped to a newer point release, or the apt sources repointed to archive.debian.org for these older security-archive packages.

System Information

N/A -- this is a CI/build infrastructure issue affecting the build
pipeline itself, not a specific mitmproxy release or version.

Checklist

• This bug affects the latest mitmproxy release.