ci: point apt at archive.debian.org so the test matrix can install

Every job in the test matrix has failed since Debian 10 "buster" was
archived, on master as well as on pull requests.

The matrix runs inside ghcr.io/zsh-users/zsh:<version>, and all 35 of
those images are buster-based -- /etc/debian_version reads 10.12 and
/etc/apt/sources.list carries:

    deb http://deb.debian.org/debian buster main
    deb http://security.debian.org/ buster/updates main

buster has since been dropped from both of those mirrors;
deb.debian.org now returns 404 for dists/buster/Release. So
install_packages -- minideb's "apt-get update && apt-get install"
helper -- fails all three of its attempts and exits 100 before
`make test` is ever reached. Nothing in the test suite is involved,
which is why the failure is uniform across every zsh version.

archive.debian.org still serves buster, including buster/updates under
debian-security, and still carries bsdmainutils, make and procps.
Repointing the two entries there restores the matrix.

The rewrite is guarded on sources.list actually mentioning buster, so
it turns into a no-op once the images are rebuilt on a supported
release instead of silently redirecting a live suite to the archive.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.github/workflows/test.yml

@@ -58,6 +58,17 @@ jobs:
       image: ghcr.io/zsh-users/zsh:${{ matrix.version }}
     steps:
       - uses: actions/checkout@v4
+      # Debian 10 "buster" -- the base of every ghcr.io/zsh-users/zsh image --
+      # is end-of-life and has been dropped from the live mirrors, so
+      # install_packages fails at apt-get update before any test can run.
+      # The buster guard keeps this a no-op once the images move to a
+      # supported release, so it cannot redirect a live suite to the archive.
+      - name: Point apt at archive.debian.org (buster is EOL)
+        run: |
+          if grep -q ' buster' /etc/apt/sources.list; then
+            sed -i 's|http://deb.debian.org/debian|http://archive.debian.org/debian|' /etc/apt/sources.list
+            sed -i 's|http://security.debian.org/|http://archive.debian.org/debian-security|' /etc/apt/sources.list
+          fi
       - run: install_packages bsdmainutils make procps
       - run: make test