Use forward slashes in PowerShell

[anderslanglands]

PowerShell consumes forward slashes in path names just fine. CMake occasionally barfs on backslashes. Therefore this PR just uses forward slashes for pwsh and everything is happy.

[linux-foundation-easycla]

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• ✅ login: anderslanglands / name: Anders Langlands (0a9f169, 03257cb)

[ColinKennedy]

Previous PR, for context: #1273

Considering that CMake apparently misbehaves even in Windows with proper, normalized paths, I'd rather normalization be an explicit, opt-in feature so that projects that require this custom behavior can still have it and the default, expected behavior still functions as expected. This PR doesn't give the user any option to opt-in / opt-out of the change. What if something else other than CMake needs backslashes? etc.

[nerdvegas]

Hey Anders,

Please see the discussion in #1302 as I think this really is the root of the issue. Feel free to add to that thread, it needs to be picked up again. But yes this stuff does need to be fixed, I just think we need to do so in a more general way (as this is not a powershell specific thing).