Copy soong variables to dist

Per aosp/2183398, out/soong/soong.variables provide some useful information for debugging purpose.
This change enables copying out/soong/soong.variables to out/dist/soong_ui/soong/soong.variables in dist mode, similar to build.ninja.gz in the same directory.
Test confirmed that the content of the out/soong/soong.variables is
equal to that of out/dist/soong_ui/soong/soong.variables.

Test: m dist $target &&
	diff out/soong/soong.variables out/dist/soong_ui/soong/soong.variables
Change-Id: I1bfa53a8e116c49fda2cc9a7252f4da0c59ce5b5
diff --git a/ui/build/soong.go b/ui/build/soong.go
index 519506e..3920ddd 100644
--- a/ui/build/soong.go
+++ b/ui/build/soong.go
@@ -502,6 +502,7 @@
 	}
 
 	distGzipFile(ctx, config, config.SoongNinjaFile(), "soong")
+	distFile(ctx, config, config.SoongVarsFile(), "soong")
 
 	if !config.SkipKati() {
 		distGzipFile(ctx, config, config.SoongAndroidMk(), "soong")