Correctly detect powershell when ssh to windows machine.

[hetong07]

Description

When ssh to a windows machine, the Detect() will return powershell.exe rather than powershell. It is due to the fact that the os.getenv() returns the absolute path to powershell rather then an empty string, which causes this issue.

fmt.Printf("%v\n",os.Getenv("SHELL"))
c:\windows\system32\windowspowershell\v1.0\powershell.exe

Related issue(s)

• Failing test: TestForceSystemdFlag on Windows Docker kubernetes/minikube#10626
• Failing test: TestFunctional/DockerEnv on Windows Docker kubernetes/minikube#10627