Add --multitree-build flag

Add a flag that enables logic specific to multitree-build builds.

Bug: b/266730267
Test: treehugger, manual
Change-Id: Ia6f392525faa9d66bb6e0692b2a2e9c3b8493b0d
diff --git a/ui/build/config_test.go b/ui/build/config_test.go
index 940d85c..1749cd4 100644
--- a/ui/build/config_test.go
+++ b/ui/build/config_test.go
@@ -903,6 +903,15 @@
 			tidyOnly:     "",
 			expectedArgs: []string{},
 		}, {
+			description:  "multitree build action executed at root directory",
+			dirsInTrees:  []string{},
+			buildFiles:   []string{},
+			rootSymlink:  false,
+			args:         []string{"--multitree-build"},
+			curDir:       ".",
+			tidyOnly:     "",
+			expectedArgs: []string{"--multitree-build"},
+		}, {
 			description:  "build action executed at root directory in symlink",
 			dirsInTrees:  []string{},
 			buildFiles:   []string{},