use pwsh as entrypoint for step scripts instead of powershell

[julmb]

The current implementation looks for an entrypoint named powershell, which exists in images like mcr.microsoft.com/powershell:windowsservercore-ltsc2022, but not in mcr.microsoft.com/powershell:nanoserver-ltsc2022, where the cross-platform variant pwsh is used. Because of this, only images based on windowsservercore can be used.

The windowsservercore-based image is 5.37GB, while the nanoserver-based image is 553MB, saving a significant amount of space.

The windowsservercore-based images also offer the pwsh command, so by using pwsh as entrypoint instead of powershell, both nanoserver-based and windowsservercore-based images can be used.

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[julmb]

It seems the situation is more complicated than I thought. Here are several packages and what shells they contain:

mcr.microsoft.com/windows/nanoserver:ltsc2022: cmd
mcr.microsoft.com/windows/servercore:ltsc2022: cmd, powershell
mcr.microsoft.com/powershell:nanoserver-ltsc2022: cmd, pwsh
mcr.microsoft.com/powershell:windowsservercore-ltsc2022: cmd, powershell, pwsh
mcr.microsoft.com/dotnet/framework/sdk:4.8: cmd, powershell
mcr.microsoft.com/dotnet/sdk:5.0: cmd, pwsh

WIth how inconsistent this is, maybe the best would be to add the option to configure which shell is used for each step? This could also be useful for operating systems other than windows, where some people might want to use shell-specific features.