netns-isolation: fix routing issues due to netns restarting

Previously, restarting a service implied restarting its netns.
For unknown reasons, this sometimes caused the netns-local address
to not be routable from the root netns for up to 20 s.
I.e., the service was sometimes unreachable after restarting.

Now the netns is no longer stopped when the service is stopped.

modules/netns-isolation.nix

@@ -164,9 +164,8 @@ in {
         "netns-${n}" = rec {
           requires = [ "nb-netns-bridge.service" ];
           after = [ "nb-netns-bridge.service" ];
-          bindsTo = [ "${n}.service" ];
-          requiredBy = bindsTo;
-          before = bindsTo;
+          requiredBy = [ "${n}.service" ];
+          before = requiredBy;
           script = ''
             ${ip} netns add ${netnsName}
             ${ipNetns} link set lo up