Speed up Azure CI installing Windows dependencies
There is known issue where PowerShell is unreasonably slow downloading
files due to an issue with rendering the progress bar, see this [issue](https://github.com/PowerShell/PowerShell/issues/2138)
That issue is fixed in PowerShell Core (available in Azure Pipelines as
pwsh.exe) but it can also be worked around by setting:
$ProgressPreference = 'SilentlyContinue'
I measured downloading LLVM and it took about 220s before, 5s after, so the improvement is significant.
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