Disable SSH connection sharing.

If you perform a really large number of concurrent builds, you run into
"broken pipe" issues and SSH connections seem to be delayed for up to 20 minutes
in my experiments. I don't know exactly what limit we hit.

We could perhaps set up multiple remote Nix builders, each pointing to
eu.nixbuild.net, but using different control sockets and having a limited
max nix jobs setting. That way the number of sessions per socket would be
limited. However, nix will probably open more sessions then just one per build,
(to query for valid paths, fetch outputs etc) so it would still be difficult to
know how many concurrent sessions each socket would receive.

nixbuild-action.sh

@@ -30,9 +30,7 @@ Host eu.nixbuild.net
   LogLevel ERROR
   StrictHostKeyChecking yes
   UserKnownHostsFile $SSH_KNOWN_HOSTS_FILE
-  ControlMaster auto
-  ControlPersist 300
-  ControlPath ~/.ssh/nixbuild_socket
+  ControlPath none
 EOF