Make NFS mount async to improve latency

Don't wait to flush writes, as we don't care about consistency on a
crash:

> This option allows the NFS server to violate the NFS protocol and
> reply to requests before any changes made by that request have been
> committed to stable storage (e.g. disc drive).
>
> Using this option usually improves performance, but at the cost that
> an unclean server restart (i.e. a crash) can cause data to be lost or
> corrupted.

Reviewed-by: Aleix Boné <abonerib@bsc.es>

m/apex/nfs.nix

@@ -7,7 +7,7 @@
     mountdPort = 4002;
     statdPort = 4000;
     exports = ''
-      /home 10.0.40.0/24(rw,sync,no_subtree_check,no_root_squash)
+      /home 10.0.40.0/24(rw,async,no_subtree_check,no_root_squash)
     '';
   };
   networking.firewall = {