Fixing build scripts to use pwsh and to call GenerateMetadataSource.ps1, rather than .cmd

[tannergooding]

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[tannergooding]

Failures are in collecting artifacts as part of CI and a failure introduced in test-win32metadata that is external to any changes made in ClangSharp:

D:\a\1\s\artifacts\win32metadata\generation\emitter\obj\generated\WinRT.modified.cs(3398,17): error CS0101: The namespace 'Windows.Win32.WinRT' already contains a definition for 'RoErrorReportingFlags'
D:\a\1\s\artifacts\win32metadata\generation\emitter\obj\generated\Dhcp.modified.cs(1268,210): error CS0246: The type or namespace name '__AnonymousRecord_dhcpsapi_L1229_C5' could not be found (are you missing a using directive or an assembly reference?)
D:\a\1\s\artifacts\win32metadata\generation\emitter\obj\generated\Dhcp.modified.cs(1282,210): error CS0246: The type or namespace name '__AnonymousRecord_dhcpsapi_L1243_C5' could not be found (are you missing a using directive or an assembly reference?)
D:\a\1\s\artifacts\win32metadata\generation\emitter\obj\generated\Dhcp.modified.cs(1297,210): error CS0246: The type or namespace name '__AnonymousRecord_dhcpsapi_L1260_C5' could not be found (are you missing a using directive or an assembly reference?)