Change Test-DotnetNewTemplates' nugetCacheRoot to C:\

[lauren-ciha]

Resolves the recent pipeline errors in #6583.

In Test-DotnetNewTemplates.ps1, a temporary NuGet cache is created to keep test template dependencies separate from the user's NuGet configuration. Originally, this path was set within the repo's path. When files were added to this path, it exceeded the Windows MAX_PATH of 260 characters. The WinUI Xaml compiler resolves transitive dependencies using APIs that are not long-path aware. Therefore, when the dependency versions were longer and the nuget cache path was deep, the total path length was longer than the maximum path length, causing the XAML compiler to not emit App.g.i.cs. When the templates were built, the missing App.g.i.cs caused a CS2001 build error. We can prevent this build error by setting the test NuGet cache path to a shorter string. Currently, this is set to $env:TEMP, but this is not the only path option. $env:TEMP was chosen because it's a short path name that is already a location for temporary files like the test cache.

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[lauren-ciha]

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[lauren-ciha]

Closing this PR because we chose another solution