Possible improvements to GitHub Actions (CI)

[YoshiRulz]

make final build downloadable

• #3945

logs can't be read without signing in (probably hardcoded site-wide)

something better than installing .NET SDK in Ubuntu

• https://devblogs.microsoft.com/dotnet/dotnet-loves-github-actions/
• https://nixos.org/guides/continuous-integration-github-actions.html
• https://github.com/Elskom/build-dotnet/

misc.

• https://github.com/neysofu/awesome-github-actions-runners#list-of-providers
• #3944

see also #3013, #3426