Always keep rsp files

It's painful to debug without these.

Test: do a build; find out -name '*.rsp'
Change-Id: I6fc25095442178c0bcbc1dea444ba51e9259c600

ui/build/ninja.go

@@ -38,6 +38,7 @@ func runNinja(ctx Context, config Config) {
 	executable := config.PrebuiltBuildTool("ninja")
 	args := []string{
 		"-d", "keepdepfile",
+		"-d", "keeprsp",
 		"--frontend_file", fifo,
 	}