Use versioned tarball URL for chkrootkit.

Pointing at the latest version is fragile: Hash updates don't
follow source updates automatically.

pkgs/tools/security/chkrootkit/default.nix

···
   name = "chkrootkit-0.50";
 
   src = fetchurl {
-    url = ftp://ftp.pangeia.com.br/pub/seg/pac/chkrootkit.tar.gz;
+    url = ftp://ftp.pangeia.com.br/pub/seg/pac/chkrootkit-0.50.tar.gz;
     sha256 = "1ivclp7ixndacjmf7xgj8lfa6h7ihx44mzzsapqdvf0c5f9gqj4m";
   };