Revert "nix.spec.in: Build from the .tar.xz file."

This reverts commit f32c6ed873. It
breaks the RPM builds because Nixpkgs's rpmBuild function only copies
in the .tar.gz file.

nix.spec.in

@@ -10,7 +10,7 @@ License: LGPLv2+
 Group: Applications/System
 %endif
 URL: http://nixos.org/
-Source0: %{name}-%{version}.tar.xz
+Source0: %{name}-%{version}.tar.bz2
 %if 0%{?el5}
 BuildRoot: %(mktemp -ud %{_tmppath}/%{name}-%{version}-%{release}-XXXXXX)
 %endif