Fix: Pipeline dependency on deb.debian.org, replace with archive

This should fix some tests, as a lot of them are dependent on
deb.debian.org which no longer supports Debian 9 "Stretch".
Instead we use archive.debian.org which will continue to contain
"Stretch" packages for a long time.

system/t04_mirror/ListMirror6Test_gold

@@ -1,7 +1,7 @@
 [
   {
     "Name": "mirror1",
-    "ArchiveRoot": "http://cdn-fastly.deb.debian.org/debian/",
+    "ArchiveRoot": "http://archive.debian.org/debian-archive/debian/",
     "Distribution": "stretch",
     "Components": [
       "main",
@@ -46,7 +46,7 @@
   },
   {
     "Name": "mirror2",
-    "ArchiveRoot": "http://cdn-fastly.deb.debian.org/debian/",
+    "ArchiveRoot": "http://archive.debian.org/debian-archive/debian/",
     "Distribution": "stretch",
     "Components": [
       "contrib"
@@ -89,7 +89,7 @@
   },
   {
     "Name": "mirror3",
-    "ArchiveRoot": "http://cdn-fastly.deb.debian.org/debian/",
+    "ArchiveRoot": "http://archive.debian.org/debian-archive/debian/",
     "Distribution": "stretch",
     "Components": [
       "non-free"