Use macro for pid file location

Previously a warning was given:

/usr/lib/systemd/user/aklog.service:9: PIDFile= path is not absolute, ignoring: ${XDG_RUNTIME_DIR}/krenew.pid

Replace this with the macro '%t' , see man systemd.service which expanded to the same thing and presumably verify is smart enough to know this will be a result.

End result after patch:

● aklog.service - Set up  AFS tokens and renew Kerberos
   Loaded: loaded (/usr/lib/systemd/user/aklog.service; enabled; vendor preset: enabled)
  Drop-In: /etc/systemd/user/aklog.service.d
           └─kdestroy.conf
   Active: active (running) since Tue 2022-07-05 10:49:20 CEST; 6min ago
  Process: 1590359 ExecStartPre=/usr/bin/aklog (code=exited, status=0/SUCCESS)
 Main PID: 1590375 (krenew)
   CGroup: /user.slice/user-16568.slice/user@16568.service/aklog.service
           └─1590375 /usr/bin/krenew -a -K 30 -i -v -L -t -p /run/user/16568/krenew.pid

as expected and:

$ systemctl show --user aklog -p PIDFile
PIDFile=/run/user/16568/krenew.pid