MT#61447 ec2: skip additional mirror for arm64 on jessie

There's no support for arm64 on
http://archive.debian.org/debian-security/dists/jessie/updates/main/
so we need to skip it, otherwise creating/updating
/var/cache/pbuilder/base-jessie-arm64.cow fails hard.

Change-Id: I62c71654ae56d0a73bd19ca3ec0f05232d333aad

ec2/bootstrap.sh

@@ -248,7 +248,14 @@ case "$distribution" in
     # nowadays resides on archive
     MIRRORSITE="http://archive.debian.org/debian/"
     # security and updates
-    OTHERMIRROR="deb http://archive.debian.org/debian-security ${distribution}/updates main"
+    case "${arch}" in
+      arm64)
+        :  # no arm64 support, so do not define additional mirror
+      ;;
+      *)
+        OTHERMIRROR="deb http://archive.debian.org/debian-security ${distribution}/updates main"
+      ;;
+    esac
     # we need key id CBF8D6FD518E17E1
     DEBOOTSTRAPOPTS=("${DEBOOTSTRAPOPTS[@]}" "--keyring=/usr/share/keyrings/debian-archive-removed-keys.gpg")
     # support bootstrapping archived repository with expired GPG key