GitHub Actions for MiniScaffold itself

[rojepp]

Proposed Changes

This builds MiniScaffold itself with GitHub Actions on Ubuntu, MacOS and Windows.
It's the first step of #143.
Test Run
The next step is to have the templates do the same.
GitHub actions is in beta and is scheduled to be released Nov 13. One can apply for the beta here. There is no waiting list any longer according to Nat Friedman.

There is a workaround in the workflow right now because of a bug in setup-dotnet. PR here. Once the bug is fixed, the DOTNET_ROOT line below can be removed in two places.

      env:
        # Work around https://github.com/actions/setup-dotnet/issues/29
        DOTNET_ROOT: ${{ runner.tool_cache }}/dncs/${{ matrix.dotnet }}/x64

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• Build and tests pass locally
• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have added necessary documentation (if appropriate)

Further comments

This PR includes the changes in #144, so if those changes aren't acceptable I'll have some rebasing to do. 😅

[TheAngryByrd]

If I just opt into the actions beta and these files exist, it should just work?

[rojepp]

@TheAngryByrd That's the plan anyway. ;-) I know there have been some issues around getting Actions to build on forks. Let me know when you've signed up, we'll see what happens.

[rojepp]

@TheAngryByrd https://twitter.com/natfriedman/status/1174158823877042177

[TheAngryByrd]

the page doesn’t want to load. I’ve submitted an issue to GitHub so we’ll see where that ends up. Thanks for the work 😊

[rojepp]

This link doesn't work? https://github.com/features/actions/signup/?account=TheAngryByrd @TheAngryByrd

[rojepp]

@TheAngryByrd Ah I just now saw that it actually built.

[TheAngryByrd]

So my script blocker was blocking the page :), LGTM now