Fix bad_format_string error when builder stdout contains % (#10063)

(cherry picked from commit e2b6821)

Co-authored-by: roblabla <[email protected]>

src/libutil/serialise.cc

@@ -444,7 +444,7 @@ Error readError(Source & source)
     auto msg = readString(source);
     ErrorInfo info {
         .level = level,
-        .msg = hintformat(fmt("%s", msg)),
+        .msg = hintfmt(msg),
     };
     auto havePos = readNum<size_t>(source);
     assert(havePos == 0);
@@ -453,7 +453,7 @@ Error readError(Source & source)
         havePos = readNum<size_t>(source);
         assert(havePos == 0);
         info.traces.push_back(Trace {
-            .hint = hintformat(fmt("%s", readString(source)))
+            .hint = hintfmt(readString(source))
         });
     }
     return Error(std::move(info));