ci: fix Linux build after Debian 11 EOL

.github/workflows/build.yml

@@ -146,9 +146,23 @@ jobs:
 
       - name: Install dependencies
         run: |
-          dpkg --add-architecture i386
-          apt-get update
-          apt-get install -y \
+          # Debian 11 reached end of LTS on 2026-08-31: the bullseye-security
+          # suite is frozen and its pool is being pruned from deb.debian.org.
+          # Keep the same toolchain, but fetch the base suite from the Debian
+          # archive and pin the security suite to its last complete snapshot.
+          # The security suite is fetched from the cloudflare snapshot mirror:
+          # plain snapshot.debian.org throttles CI bursts with connection
+          # resets, and https needs ca-certificates, which debian:11-slim
+          # does not ship, so bootstrap it from the archive first.
+          cat > /etc/apt/sources.list <<'EOF'
+          deb http://archive.debian.org/debian bullseye main
+          deb http://archive.debian.org/debian bullseye-updates main
+          EOF
+          apt-get -o Acquire::Check-Valid-Until=false update
+          apt-get install -y --no-install-recommends ca-certificates
+          echo 'deb https://snapshot-cloudflare.debian.org/archive/debian-security/20260831T000000Z bullseye-security main' > /etc/apt/sources.list.d/bullseye-security.list
+          apt-get -o Acquire::Check-Valid-Until=false -o Acquire::Retries=5 update
+          apt-get -o Acquire::Retries=5 install -y \
             gcc-multilib g++-multilib \
             build-essential \
             libc6-dev libc6-dev-i386 \