Switch CI to GitHub Actions

[giordano]

This is still a work-in-progress

[giordano]

@SaschaMann either I'm doing something wrong, or the prefix doesn't seem to work:

Run julia-actions/julia-runtest@prefix
  with:
    inline: yes
    coverage: true
    depwarn: yes
    project: @.

For the record, xvfb and xauth are already installed in the workers environments

Edit: nevermind, this seems to work well instead

[giordano]

So much more green now than one hour ago! Only jobs on nightly are failing.

In order for documentation deployment to actually work, someone needs to setup the SSH deploy key for GitHub Actions

[tknopp]

I added the keys, please ping me once this is ready to go.

[giordano]

Good to go now!

Note that you may want to disable AppVeyor (in AppVeyor settings set the build to off) and remove its webhook in the repository settings