networkmanager-fortisslvpn: create the local state directory

Otherwise connecting simply fails:
VPN connection: failed to connect: 'La création du fichier « /tmp/lib/NetworkManager-fortisslvpn/0507e3ef-f0e0-4153-af64-b3d9a025877c.config.XSB19Z » a échoué : No such file or directory'

nixos/modules/services/networking/networkmanager.nix

@@ -472,6 +472,7 @@ in {
     systemd.tmpfiles.rules = [
       "d /etc/NetworkManager/system-connections 0700 root root -"
       "d /etc/ipsec.d 0700 root root -"
+      "d /var/lib/NetworkManager-fortisslvpn 0700 root root -"
 
       "d /var/lib/dhclient 0755 root root -"
       "d /var/lib/misc 0755 root root -" # for dnsmasq.leases

pkgs/tools/networking/network-manager/fortisslvpn/default.nix

@@ -32,10 +32,14 @@ in stdenv.mkDerivation {
   configureFlags = [
     "--without-libnm-glib"
     "--with-gnome=${if withGnome then "yes" else "no"}"
-    "--localstatedir=/tmp"
+    "--localstatedir=/var"
     "--enable-absolute-paths"
   ];
 
+  # the installer only create an empty directory in localstatedir, so
+  # we can drop it
+  installFlags = [ "localstatedir=." ];
+
   passthru = {
     updateScript = gnome3.updateScript {
       packageName = pname;