Pin squeeze-updates high enough so that its packages may be installed.

Until then, our APT pinning made packages from there disregarded in favor of
older Squeeze ones.

Incidentally, this also used to break the build,
since lsb-printing depends on "at",
and the current sid kernel conflicts with Squeeze's "at", but does not conflict
with squeeze-updates' "at".

config/chroot_apt/preferences

@@ -303,6 +303,10 @@ Package: *
 Pin: origin mozilla.debian.net
 Pin-Priority: 991
 
+Package: *
+Pin: release o=Debian,n=squeeze-updates
+Pin-Priority: 990
+
 Package: *
 Pin: release o=Debian,n=squeeze
 Pin-Priority: 900