Add GitHub actions/setup-dotnet support

[lahma]

More often than I want, I seem to hit the oddities of different GH runner images. sometimes they have both NET 8 and NET 9, sometimes only either one. NUKE's setup seems to force the one in global JSON and then when some component explicitly would want to use the other version it's missing (because installation replaces the older one - if installing 9, 8 will be removed).

This PR adds support for specifying versions of .NET that should all be present.

I confirm that the pull-request:

• [x] Follows the contribution guidelines
• [x] Is based on my own work
• [x] Is in compliance with my employer

[matkoch]

DM :)