plausible: fix admin user password seed and SMTP passwords

LoadCredential was misused as it is not building any environment variable,
it is the responsibility of our preStart to do it so
Plausible's script can pick it up.

nixos/modules/services/web-apps/plausible.nix

@@ -238,9 +238,12 @@ in {
           path = [ cfg.package ]
             ++ optional cfg.database.postgres.setup config.services.postgresql.package;
           script = ''
-            export CONFIG_DIR=$CREDENTIALS_DIRECTORY
-
             export RELEASE_COOKIE="$(< $CREDENTIALS_DIRECTORY/RELEASE_COOKIE )"
+            export ADMIN_USER_PWD="$(< $CREDENTIALS_DIRECTORY/ADMIN_USER_PWD )"
+            export SECRET_KEY_BASE="$(< $CREDENTIALS_DIRECTORY/SECRET_KEY_BASE )"
+
+            ${lib.optionalString (cfg.mail.smtp.passwordFile != null)
+              ''export SMTP_USER_PWD="$(< $CREDENTIALS_DIRECTORY/SMTP_USER_PWD )"''}
 
             # setup
             ${cfg.package}/createdb.sh