Fix CI: Use archive.debian.org for Python 2.7 Buster image

Debian Buster reached end-of-life and its repositories are no longer
available at deb.debian.org. This updates the CI to use archive.debian.org
for the Python 2.7 build which requires the Buster image.

Author: sgd2z

.github/workflows/ci.yaml

@@ -30,10 +30,10 @@ jobs:
         if: matrix.image-variant == '-buster'
         run: |
           # Debian Buster reached EOL, switch to archive.debian.org
-          # Remove security repo entirely (not available in archive) and use main archive
           sed -i 's|deb.debian.org|archive.debian.org|g' /etc/apt/sources.list
-          sed -i '/security.debian.org/d' /etc/apt/sources.list
+          sed -i 's|security.debian.org|archive.debian.org|g' /etc/apt/sources.list
           sed -i '/buster-updates/d' /etc/apt/sources.list
+          sed -i 's|buster/updates|buster|g' /etc/apt/sources.list
 
       - name: Install python dependencies
         run: |