ImmutableMap.Builder.build() is deprecated (#209)

... and we probably want buildKeepingLast to avoid unnecessary brittleness.

Fixes internal linter complaints.
diff --git a/src/main/java/com/google/devtools/build/skydoc/rendering/Stamping.java b/src/main/java/com/google/devtools/build/skydoc/rendering/Stamping.java
index be7f5f3..2da4eb5 100644
--- a/src/main/java/com/google/devtools/build/skydoc/rendering/Stamping.java
+++ b/src/main/java/com/google/devtools/build/skydoc/rendering/Stamping.java
@@ -42,7 +42,7 @@
       String[] kv = line.split(" ", 2); // split on first space only
       builder.put(kv[0], kv[1]);
     }
-    return builder.build();
+    return builder.buildKeepingLast();
   }
 
   private Stamping(