debian: automatically accept stable -> oldstable change

apt as released in initial debian-10 template, requires confirming the
repository change from stable to oldstable (which happened with
debian-11 release). Later versions of apt has this fixes, but lets fix
updating from the older version too.

Simply call 'apt-get update --allow-releaseinfo-change' before the
update.

Related to QubesOS/qubes-issues#6624
Fixes QubesOS/qubes-issues#5149 (which was about the very same thing
with previous debian version)

update/qubes-vm.sls

@@ -19,6 +19,14 @@ dnf-makecache:
     - stateful: True
 {% endif %}
 
+{% if grains['oscodename'] == 'buster' %}
+# https://bugs.debian.org/931566
+# Apply the workaround manually, to be able to pull in the fixed apt version
+apt-get update --allow-releaseinfo-change:
+  cmd.run:
+   - order: 1
+{% endif %}
+
 update:
   pkg.uptodate:
     - refresh: True