[bug] incorrect output for vela cli installation

install vela cli using https://kubevela.io/script/install.sh, the output like:

```
Installing v1.2.0-rc.2 Vela CLI...
Downloading https://github.com/oam-dev/kubevela/releases/download/v1.2.0-rc.2/vela-v1.2.0-rc.2-darwin-amd64.tar.gz ...
cp: /usr/local/bin/vela: Permission denied
vela installed into /usr/local/bin successfully.
A Highly Extensible Platform Engine based on Kubernetes and Open Application Model.
```

i have old version vela-cli at /usr/local/bin/vela, and the cp command was failed by `Permission denied`, but it still output `vela installed into /usr/local/bin successfully.`

Start with https://kubevela.io/script/install.sh and inspect the installation output around the copy into /usr/local/bin. Reproduce the install with a non-writable destination, then verify that the success message appears only when the copy succeeds and that the existing vela binary is handled correctly.

Written by the indexing model from the issue text.