sql: fix small race in distIndexBackfiller

This fixes a small race condition in
distIndexBackfiller. updateJobDetails calls SetResumeSpansInJob which
mutates the ResumeSpanList in the job details.  Normally, this is only
called from the periodic updater.  However, when the testing knob
AlwasyUpdateIndexBackfillDetails is set, we also update it on every
ProducerMetadata message we get back

Release note: None

pkg/sql/backfill.go

@@ -1077,9 +1077,16 @@ func (sc *SchemaChanger) distIndexBackfill(
 		})
 	}
 
+	// updateJobMu ensures only one goroutine is calling
+	// updateJobDetails at a time to avoid a data race in
+	// SetResumeSpansInJob. This mutex should be uncontended when
+	// sc.testingKnobs.AlwaysUpdateIndexBackfillDetails is false.
+	var updateJobMu syncutil.Mutex
 	updateJobDetails = func() error {
 		updatedTodoSpans := getTodoSpansForUpdate()
 		return sc.db.Txn(ctx, func(ctx context.Context, txn *kv.Txn) error {
+			updateJobMu.Lock()
+			defer updateJobMu.Unlock()
 			// No processor has returned completed spans yet.
 			if updatedTodoSpans == nil {
 				return nil