Install xul-ext-noscript from wheezy-backports.

The version in there is good enough for the version of Iceweasel we
currently ship. Besides, it's the same as in sid as of today.

So, no need to track unstable on that one.

config/chroot_apt/preferences

@@ -226,7 +226,7 @@ Pin: release o=Debian,a=unstable
 Pin-Priority: 999
 
 Package: xul-ext-noscript
-Pin: release o=Debian,a=unstable
+Pin: release o=Debian Backports,n=wheezy-backports
 Pin-Priority: 999
 
 Explanation: override deb.tails.boum.org's Squeeze -specific package.