Use uppercase for `OS` env variable

This patch fixes issue #9213.

I learnt that `os.environ` object in Windows uppercases all keys:
http://stackoverflow.com/a/7797329/441652
In fact, all environment variables in Windows are case-insitive. For example, the following commands refer to the same environment variable:
```
echo %os%
echo %OS%
```

However, when `salt-ssh` is used with Windows/Cygwin where Python is "natively" compiled under Cygwin, `os.environ` object apparently does not perform uppercasing anymore. And command `salt-ssh` fails for Cygwin.

The change will fix #9213 issue without affecting Windows-only nodes.