add the bullseye-fasttrack repository

Currently we are going to need it only for virtualbox in one host,
but I preferred to be more generic and add it everywhere for
consistency, since it's currently well maintained.

Signed-off-by: Mattia Rizzolo <mattia@debian.org>

hosts/common/etc/apt/sources.list

@@ -9,3 +9,5 @@ deb http://security.debian.org/debian-security bullseye-security main contrib no
 
 deb http://deb.debian.org/debian/ bullseye-backports main contrib non-free
 #deb-src http://deb.debian.org/debian/ bullseye-backports main contrib non-free
+
+deb https://fasttrack.debian.net/debian/ bullseye-fasttrack main contrib non-free