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

The Nix download page only mentions the .xz source tarball, so that's what
people are likely to have available. This means that somebody who downloads a
Nix source tarball can turn it directly into an RPM with `rpmbuild -ta
nix-*.tar.xz`.

nix.spec.in

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