apt: Use salt buster repo for bullseye for now, as there is none for bullseye yet

Signed-off-by: Maximilian Wilhelm <max@sdn.clinic>

apt/salt.list.Debian.bullseye

@@ -0,0 +1 @@
+deb http://repo.saltstack.com/py3/debian/10/amd64/3002/ buster main