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/drop.py

@@ -6,7 +6,7 @@ class DropMirror1Test(BaseTest):
     drop mirror: regular list
     """
     fixtureCmds = [
-        "aptly mirror create --ignore-signatures mirror1 http://cdn-fastly.deb.debian.org/debian/ stretch",
+        "aptly mirror create --ignore-signatures mirror1 http://archive.debian.org/debian-archive/debian/ stretch",
     ]
     runCmd = "aptly mirror drop mirror1"