nixos/frr: make runtime directory world-readable

FRR intends for non-root users to connect to the VTY sockets if they
are members of the frrvty group, however this is not possible if
non-root/non-frr users cannot access the runtime directory. The
sockets used by the FRR daemons for internal IPC are also created in
the runtime directory, however these are created with appropriately
restrictive permissions to prevent interference.

nixos/modules/services/networking/frr.nix

@@ -220,7 +220,7 @@ in
         '';
       };
 
-      systemd.tmpfiles.rules = [ "d /run/frr 0750 frr frr -" ];
+      systemd.tmpfiles.rules = [ "d /run/frr 0755 frr frr -" ];
 
       systemd.services.frr = {
         description = "FRRouting";