ui: fix logic sense for multitree build

Bug: b/266730267
Test: treehugger, manual
Change-Id: I059a9b90ecb70de5965cb2fe3f8a1e16de46c116
diff --git a/ui/build/soong.go b/ui/build/soong.go
index 655ae35..871e637 100644
--- a/ui/build/soong.go
+++ b/ui/build/soong.go
@@ -168,7 +168,7 @@
 		commonArgs = append(commonArgs, "-t")
 	}
 
-	if !pb.config.multitreeBuild {
+	if pb.config.multitreeBuild {
 		commonArgs = append(commonArgs, "--multitree-build")
 	}