Add warning about out-directory usage with RBE builds

Prints the following warning messsage:
WARNING:
Setting OUT_DIR to a path other than out may result in slow RBE builds.
See http://go/android_rbe_out_dir for a workaround.

Bug: b/169676232
Change-Id: Id1ad06c4dc672c24373642e0b624833eb6a0dbcf
diff --git a/cmd/soong_ui/main.go b/cmd/soong_ui/main.go
index 69e4f69..5c06251 100644
--- a/cmd/soong_ui/main.go
+++ b/cmd/soong_ui/main.go
@@ -173,6 +173,7 @@
 	rbeMetricsFile := filepath.Join(logsDir, c.logsPrefix+"rbe_metrics.pb")
 	soongMetricsFile := filepath.Join(logsDir, c.logsPrefix+"soong_metrics")
 	defer build.UploadMetrics(buildCtx, config, c.simpleOutput, buildStarted, buildErrorFile, rbeMetricsFile, soongMetricsFile)
+	build.PrintOutDirWarning(buildCtx, config)
 
 	os.MkdirAll(logsDir, 0777)
 	log.SetOutput(filepath.Join(logsDir, c.logsPrefix+"soong.log"))