fix variable interpolation in service description

Signed-off-by: ahuston-0 <aliceghuston@gmail.com>

modules/verify.nix

@@ -73,7 +73,9 @@ in
         lib.nameValuePair "nix-verifiers@${service-name}" {
           requires = [ "multi-user.target" ];
           after = [ "multi-user.target" ];
-          description = "Verify nix store (verify-contents: ${verify-contents}, verify-trust: ${verify-trust}, signatures-needed: ${signatures-needed})";
+          description =
+            "Verify nix store (verify-contents: ${lib.boolToString verify-contents}, verify-trust: "
+            + "${lib.boolToString verify-trust}, signatures-needed: ${builtins.toString signatures-needed})";
           serviceConfig = {
             Type = "oneshot";
             User = "root";