Make file does not exist condition more clear

nixos/modules/issuer.nix

@@ -173,13 +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.
-      #
-      # https://www.freedesktop.org/software/systemd/man/systemd.service.html#Command%20lines
-      # > If the executable path is prefixed with "+" then the process is
-      # > executed with full privileges.
-      serviceConfig.ExecStartPre = [
-        "+${pkgs.bash}/bin/bash -c '[ ! -f /var/db/vouchers.sqlite3 ]'"
-      ];
+      unitConfig.ConditionPathExists = "!/var/db/vouchers.sqlite3";
 
       script =
         let