nixos/networkmanager: create pppd lock directory

I digged up some 3G stick, which uses ppp to set up the connection.

It failed to spin up ppp, because ppp failed to find the directory it wants to create its lockfiles in:

```
Jul 22 16:47:49 tp ModemManager[926779]: <info>  [modem1] state changed (connected -> disconnecting)
Jul 22 16:47:49 tp ModemManager[926779]: <info>  [modem1] simple connect started...
Jul 22 16:47:49 tp ModemManager[926779]: <info>  [modem1] simple connect state (4/10): wait to get fully enabled
Jul 22 16:47:50 tp ModemManager[926779]: <info>  [modem1] state changed (disconnecting -> registered)
Jul 22 16:47:50 tp ModemManager[926779]: <info>  [modem1] simple connect state (5/10): wait after enabled
Jul 22 16:47:50 tp ModemManager[926779]: <info>  [modem1/bearer0] connection NixOS#11 finished: duration 1s
Jul 22 16:47:50 tp ModemManager[926779]: <info>  [modem1] simple connect state (6/10): register
Jul 22 16:47:50 tp ModemManager[926779]: <info>  [modem1] simple connect state (7/10): wait to get packet service state attached
Jul 22 16:47:50 tp ModemManager[926779]: <info>  [modem1] simple connect state (8/10): bearer
Jul 22 16:47:50 tp ModemManager[926779]: <info>  [modem1] simple connect state (9/10): connect
Jul 22 16:47:50 tp ModemManager[926779]: <info>  [modem1] state changed (registered -> connecting)
Jul 22 16:47:50 tp ModemManager[926779]: <info>  [modem1] state changed (connecting -> connected)
Jul 22 16:47:50 tp ModemManager[926779]: <info>  [modem1] simple connect state (10/10): all done
Jul 22 16:47:50 tp pppd[1576260]: Plugin /nix/store/yqdqzz6y6agcmrfj8b6pwqhjcjyb3ypr-networkmanager-1.42.6/lib/pppd/2.5.0/nm-pppd-plugin.so loaded.
Jul 22 16:47:50 tp NetworkManager[1576260]: Plugin /nix/store/yqdqzz6y6agcmrfj8b6pwqhjcjyb3ypr-networkmanager-1.42.6/lib/pppd/2.5.0/nm-pppd-plugin.so loaded.
Jul 22 16:47:50 tp pppd[1576260]: nm-ppp-plugin: initializing
Jul 22 16:47:50 tp pppd[1576260]: pppd 2.5.0 started by root, uid 0
Jul 22 16:47:50 tp pppd[1576260]: Can't create lock file /var/run/pppd/lock/LCK..ttyUSB0: No such file or directory
Jul 22 16:47:50 tp NetworkManager[1576260]: Can't create lock file /var/run/pppd/lock/LCK..ttyUSB0: No such file or directory
Jul 22 16:47:50 tp pppd[1576260]: nm-ppp-plugin: status 2 / phase 'serial connection'
Jul 22 16:47:50 tp pppd[1576260]: Exit.
Jul 22 16:47:50 tp pppd[1576260]: nm-ppp-plugin: status 0 / phase 'dead'
Jul 22 16:47:50 tp pppd[1576260]: nm-ppp-plugin: cleaning up
```

Creating the directories via tmpfiles.d got the connection to succeed,
and might also fix other connections using PPP.

nixos/modules/services/networking/networkmanager.nix

@@ -461,6 +461,8 @@ in {
       "d /var/lib/NetworkManager-fortisslvpn 0700 root root -"
 
       "d /var/lib/misc 0755 root root -" # for dnsmasq.leases
+      # ppp isn't able to mkdir that directory at runtime
+      "d /run/pppd/lock 0700 root root -"
     ];
 
     systemd.services.NetworkManager = {