buildSpan -> singleSpanBuilder (#9995)

Fixed oversight where one overload of startSpan wasn't updated to use singleSpanBuilder

Author: dougqh

dd-trace-core/src/main/java/datadog/trace/core/CoreTracer.java

@@ -1109,7 +1109,7 @@ public AgentSpan startSpan(
       final CharSequence spanName,
       final AgentSpanContext parent,
       final long startTimeMicros) {
-    return buildSpan(instrumentationName, spanName)
+    return singleSpanBuilder(instrumentationName, spanName)
         .ignoreActiveSpan()
         .asChildOf(parent)
         .withStartTimestamp(startTimeMicros)