Pin multistrap sources to fixed tags

- The untagged source is constantly updating and often incomplete, pin
to a fixed tag to reduce flakiness.

Change-Id: Ieedaaa863397f864c50dfd8ddfa13d9f9cd306a9
diff --git a/multistrap.conf b/multistrap.conf
index e39c8e3..512f55b 100644
--- a/multistrap.conf
+++ b/multistrap.conf
@@ -9,14 +9,14 @@
 [main]
 packages=MAIN_PACKAGES
 source=https://packages.cloud.google.com/apt
-suite=aiy-debian-stretch-main
+suite=aiy-debian-stretch-main-20180923-1
 
 [non-free]
 packages=
 source=https://packages.cloud.google.com/apt
-suite=aiy-debian-stretch-non-free
+suite=aiy-debian-stretch-non-free-20180918-1
 
 [contrib]
 packages=
 source=https://packages.cloud.google.com/apt
-suite=aiy-debian-stretch-contrib
+suite=aiy-debian-stretch-contrib-20180918-1