fetch-configlet.ps1: don't require Windows

[ee7]

From https://github.com/exercism/problem-specifications/pull/1799#discussion_r653396492:

In the future, we could consider modifying the PowerShell script to also download the binaries for non-Windows OS'es (PowerShell is cross-platform nowadays).

This seems like a good idea - it's more complex to explain to users that the bash script works on macOS/Linux/Windows, but the PowerShell script is Windows-only.

Might be a good idea to test these scripts in CI first, see: https://github.com/exercism/configlet/issues/121