Enable reclient cache directory.

The cache directory stores files that should persist between builds.
Currently it will contain reproxy.creds, which caches the authentication
mechanism and token used to make subsequent builds faster.

Test: m nothing build takes 7s
Change-Id: Iaaa20b92630405918e5e91e09eca6c798b8fc872

ui/build/build.go

@@ -259,10 +259,16 @@ func Build(ctx Context, config Config) {
 		startGoma(ctx, config)
 	}
 
+	rbeCh := make(chan bool)
 	if config.StartRBE() {
 		cleanupRBELogsDir(ctx, config)
-		startRBE(ctx, config)
+		go func() {
+			startRBE(ctx, config)
+			close(rbeCh)
+		}()
 		defer DumpRBEMetrics(ctx, config, filepath.Join(config.LogsDir(), "rbe_metrics.pb"))
+	} else {
+		close(rbeCh)
 	}
 
 	if what&RunProductConfig != 0 {
@@ -315,11 +321,11 @@ func Build(ctx Context, config Config) {
 		testForDanglingRules(ctx, config)
 	}
 
+	<-rbeCh
 	if what&RunNinja != 0 {
 		if what&RunKati != 0 {
 			installCleanIfNecessary(ctx, config)
 		}
-
 		runNinjaForBuild(ctx, config)
 	}