[Windows] [ssh-runner] Powershell commands escaping

When I write C:\workspace in the commands section of a step in the ssh runner, it will be replaced by C:\workspace.

docker -v for example does not accept C:/workspace and does also not accept C:\workspace - it can be worked around by moving that invocation into a separate script.

the escaping logic can be found at https://github.com/drone/runner-go/blob/master/shell/powershell/powershell.go#L29