Fix OOM in describeNestedSetFingerprint

`describedNestedSetFingerprint` appended a `StringBuilder` to itself in a loop instead of the actual item from the nested set, resulting in OOMs in `Runfiles#describeKey` and `RepoMappingManifestAction#describeKey`.

Work towards bazelbuild#18666

Closes bazelbuild#18668.

PiperOrigin-RevId: 540270874
Change-Id: Id408ab4c2438bea264b586f8ae5567dc41260242

src/main/java/com/google/devtools/build/lib/collect/nestedset/NestedSetFingerprintCache.java

@@ -84,7 +84,7 @@ public static <T> String describedNestedSetFingerprint(
     sb.append("size: ").append(list.size()).append('\n');
     for (T item : list) {
       sb.append("  ");
-      mapFn.expandToCommandLine(item, s -> sb.append(sb).append(", "));
+      mapFn.expandToCommandLine(item, s -> sb.append(s).append(", "));
       sb.append('\n');
     }
     return sb.toString();