Make path check a stronger Assertion instead of a Condition

nixos/modules/issuer.nix

@@ -173,7 +173,7 @@ in {
       # Bail if there is still an old (root-owned) DB file on this system.
       # If you hit this, and this /var/db/ file is indeed current, move it to
       # /var/lib/zkapissuer/vouchers.sqlite3 and chown it to zkapissuer:zkapissuer.
-      unitConfig.ConditionPathExists = "!/var/db/vouchers.sqlite3";
+      unitConfig.AssertPathExists = "!/var/db/vouchers.sqlite3";
 
       script =
         let