Make linking the kubeconfig idempotent. (istio#1852)

Automatic merge from submit-queue.

Make linking the kubeconfig idempotent.

Previously there were situations where we needed to manually unlink the config, e.g., after build errors.

```release-note
NONE
```

Makefile

@@ -146,7 +146,7 @@ artifacts: docker
 	@echo 'To be added'
 
 pilot/platform/kube/config:
-	ln -s ~/.kube/config pilot/platform/kube/
+	ln -fs ~/.kube/config pilot/platform/kube/
 
 .PHONY: artifacts build checkvars clean docker test setup push
 
@@ -164,4 +164,4 @@ show.goenv: ; $(info $(H) go environment...)
 
 # show makefile variables. Usage: make show.<variable-name>
 show.%: ; $(info $* $(H) $($*))
-	$(Q) true
+	$(Q) true