Use the archive repo for Buster (sonic-net#18678)

Why I did it
Buster is EOL, and the backports section has been removed from the main Debian repos. This also means that our default mirror (for non-snapshot builds) is also affected.

Work item tracking
Microsoft ADO (number only): 27691460
How I did it
Change to using archive.debian.org directly for Buster.

How to verify it

scripts/build_mirror_config.sh

@@ -21,6 +21,10 @@ if [ "$ARCHITECTURE" == "armhf" ]; then
     DEFAULT_MIRROR_SECURITY_URLS=http://deb.debian.org/debian-security/
 fi
 
+if [ "$DISTRIBUTION" == "buster" ]; then
+    DEFAULT_MIRROR_URLS=http://archive.debian.org/debian/
+fi
+
 if [ "$MIRROR_SNAPSHOT" == y ]; then
     if [ -f "$MIRROR_VERSION_FILE" ]; then
         DEBIAN_TIMESTAMP=$(grep "^debian==" $MIRROR_VERSION_FILE | tail -n 1 | sed 's/.*==//')