nixos/sks: Fix another regression from ab5dcc70

The two directories KDB and PTree do not exist before the SKS DB is
build for the first time. If /var/db/sks is empty and the module is
enabled via "services.sks.enable = true;" the following error will
occur:
...-unit-script-sks-db-pre-start[xxx]:
ln: failed to create symbolic link 'KDB/DB_CONFIG': No such file or directory

To avoid this both links have to be created after the DB is build.
Note: Creating the directories manually might be better but the initial
build might be skipped as a result:
unit-script-sks-db-pre-start[xxxxx]: KeyDB directory already exists.  Exiting.
unit-script-sks-db-pre-start[xxxxx]: PTree directory already exists.  Exiting.