env.invoke tests fail on Windows

Tests on windows that cause `cmdix.command.env.invoke` to be invoked are failing on Windows when backslashes in the command path appear to have been lost.

For example, here are the Popen args for failing test_command_opts:

```
args = 'D:acmdixcmdix.toxpyScriptspython.EXE --help', executable = None
```

Start with cmdix.command.env.invoke and the failing test_command_opts test, then reproduce it on Windows to inspect the Popen arguments. Done means command paths retain their backslashes and the affected tests pass on Windows.

Written by the indexing model from the issue text.