Moving to Github Actions for CI

[shibumi]

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Description of issue or feature request:
Today I ran into a network problem with appveyor:

Build started
git clone -q https://github.com/in-toto/in-toto-golang.git c:\gopath\src\github.com\in-toto\in-toto-golang
fatal: unable to access 'https://github.com/in-toto/in-toto-golang.git/': Failed to connect to github.com port 443: Timed out
Command exited with code 128

Mistakes happen. This issue is not intended as a rant about appveyor, but I have read that the TUF team thinks about moving to Github Actions. Maybe it makes sense for us to switch, too. See this issue as a discussion, if we should move and if so, which benefits we would gain

[lukpueh]

We also considered using Travis for windows builds too a few months ago (see theupdateframework/python-tuf#940 (comment)).

Looking into GitHub Actions seems like a good idea though.