Send soong output to std.log in multiproduct_kati

Instead of throwing it away (though it would also be in soong.log along
with the verbose messages).

Test: build/soong/build_test.bash with ctx.Println added
Change-Id: I64f8b11ab6752a6be8a5934fb41e30439238e331

cmd/multiproduct_kati/main.go

@@ -15,7 +15,6 @@
 package main
 
 import (
-	"bytes"
 	"context"
 	"flag"
 	"fmt"
@@ -280,7 +279,7 @@ func main() {
 				log.Fatalf("Error creating std.log: %v", err)
 			}
 
-			productLog := logger.New(&bytes.Buffer{})
+			productLog := logger.New(f)
 			productLog.SetOutput(filepath.Join(productLogDir, "soong.log"))
 
 			productCtx := build.Context{&build.ContextImpl{