Windows's s3cli has `.exe` extension
Windows's s3cli has `.exe` extension

fixes:
```
error running command: no matches found for pattern: out-windows/s3cli-*-windows-amd64
```

[#131456867](https://www.pivotaltracker.com/story/show/131456867)

pivotal-golang/s3cli,pivotal-golang/s3cli,pivotal-golang/s3cli,pivotal-golang/s3cli