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.

hydra-module.nix

@@ -225,6 +225,7 @@ in
         base_uri ${cfg.hydraURL}
         notification_sender ${cfg.notificationSender}
         max_servers 25
+        compress_num_threads 0
         ${optionalString (cfg.logo != null) ''
           hydra_logo ${cfg.logo}
         ''}