Use the same mechanism to load kubeconfig

Interceptors and EL are both running in the cluster. They should share
the same way of loading the kubeconfig.

Using injection package, it allows the user to override the kubeconfig
with an env. variable.

cmd/interceptors/main.go

@@ -26,7 +26,6 @@ import (
 
 	"github.com/tektoncd/triggers/pkg/interceptors/server"
 	"go.uber.org/zap"
-	"k8s.io/client-go/rest"
 	secretInformer "knative.dev/pkg/client/injection/kube/informers/core/v1/secret"
 	"knative.dev/pkg/injection"
 	"knative.dev/pkg/logging"
@@ -45,12 +44,9 @@ func main() {
 	// set up signals so we handle the first shutdown signal gracefully
 	ctx := signals.NewContext()
 
-	clusterConfig, err := rest.InClusterConfig()
-	if err != nil {
-		log.Fatalf("Failed to build config: %v", err)
-	}
+	cfg := injection.ParseAndGetRESTConfigOrDie()
 
-	ctx, startInformer := injection.EnableInjectionOrDie(ctx, clusterConfig)
+	ctx, startInformer := injection.EnableInjectionOrDie(ctx, cfg)
 
 	zap, err := zap.NewProduction()
 	if err != nil {