Safeguard: make sure this is invoked on a Windows machine

[Ecco]

Since msys2 only makes sense on Windows, we could add a check to make sure we're on a Windows machine and insult warn the daydreaming user if it's not the case.

[Ecco]

For the record, here's how the check is done in @tool-cache:

const IS_WINDOWS = process.platform === 'win32'