Add explicit error when attempting to use automatic checksumming on non-windows

[TheCakeIsNaOH]

Checklist

• I have verified this is the correct repository for opening this issue.
• I have verified no other issues exist related to my request.

Is Your Feature Request Related To A Problem? Please describe.

When using Chocolatey-AU on non-windows systems, the automatic checksumming feature does not work, and when it fails, there is an unclear error message relating to the inability to import and use the chocolatey helpers module.

Describe The Solution. Why is it needed?

Add a check before the automatic checksumming code that will throw an error if running on a non-windows platform.

Additional Context

See majkinetor/au#234

Related Issues

No response