Use pixz instead of bzip2 for compressing nars

According to following two benchmarks:

- https://www.rootusers.com/gzip-vs-bzip2-vs-xz-performance-comparison/
- http://catchchallenger.first-world.info/wiki/Quick_Benchmark:_Gzip_vs_Bzip2_vs_LZMA_vs_XZ_vs_LZ4_vs_LZO

xz has better compression ratio than bzip2 at lowest compression rate.

https://github.com/vasi/pixz has been chosen as it can scale compressing
over multiple cores linearly.

We're using this in snabblab for a month now and it has improved CPU
wise the main Hydra server.

release.nix

@@ -134,7 +134,7 @@ rec {
         ];
 
       hydraPath = lib.makeBinPath (
-        [ libxslt sqlite subversion openssh nix coreutils findutils
+        [ libxslt sqlite subversion openssh nix coreutils findutils pixz
           gzip bzip2 lzma gnutar unzip git gitAndTools.topGit mercurial darcs gnused bazaar
         ] ++ lib.optionals stdenv.isLinux [ rpm dpkg cdrkit ] );