Add github actions CI

[IanButterworth]

I thought it would be worth seeing how CI runs on github actions, given it can be pretty slow on appveyor, and travis has recently been a bit slow to respond

One of the options mentioned in #2066

The actions won't appear here until this is merged, or this PR is replicated on a non-fork branch.
You can see them here IanButterworth#1 (the coverage processing step will fail on each on that fork)

[IanButterworth]

There appears to be some odd TEMP dir issues on windows:
https://github.com/ianshmean/Pkg.jl/runs/1303945893?check_suite_focus=true#step:5:811

Seems to be actions/runner-images#712
but the recommended fix isn't working (or perhaps done in the right way)

[DilumAluthge]

@ianshmean @KristofferC I'd recommend replacing the on: section with this:

on:
  pull_request:
    branches:
      - master
  push:
    branches:
      - master
    tags: '*'

That way, CI will actually run on pull requests. (Right now, it will not run on pull requests.)

[KristofferC]

What causes it not to run now?

[DilumAluthge]

Hmm. Are actions enabled for this repo? Go to Settings and click on Actions in the left hand sidebar.

[KristofferC]

Looks like it

[DilumAluthge]

Really strange.

[DilumAluthge]

Try merging this PR and let's see if it runs on master?

[DilumAluthge]

Also: are Actions enabled for the JuliaLang organization?

https://docs.github.com/en/free-pro-team@latest/github/setting-up-and-managing-organizations-and-teams/disabling-or-limiting-github-actions-for-your-organization

[KristofferC]

JuliaLang allows any actions as well. When I made a PR to another repo JuliaLang/TOML.jl#12 they should up straight away.

[DilumAluthge]

Might be a syntax error in the workflow.

Or a fork issue.

@KristofferC how about you:

1. Make a PR from a branch inside this repo.
2. Copy a workflow file that you have used before (e.g. add github actions CI TOML.jl#12) so we know the syntax works