GitHub Action to just test on macOS, Ubuntu, and Windows

[cclauss]

Demonstrate how to install and use just in GitHub Actions on the three major operating systems without needing third-party GitHub Actions such as https://github.com/casey/just#github-actions

Developers may be interested to run the same just commands that they use locally on continuous integration platforms such GitHub Actions. Every time that a contributor creates a pull request, just test could be run on the three major operating systems to provide feedback to both the contributor and reviewers that tests are passing.

[casey]

This is dope! I'm not sure we actually want to run it though as part of our own actions runs, since it will add another job when you make a PR, etc, which is already a little noisy with five jobs. How about moving this to a contrib directory? It's pretty common to have a directory called contrib, which contains extras like these. I just merged #1870, which creates a contrib directory. It can be called something like contrib/github-actions-install-just.yaml.

[cclauss]

Can we please run it once (to make sure it passes) before we move it into the docs somewhere?

If you merge one of the other two PRs then I update branch (below) then this action will run and we can see if all the polyglot languages are present on all three OSes.

[casey]

Yup, for sure. I think it just ran and it looks like it was successful.

[cclauss]

OK... Should we move this code into https://github.com/casey/just/blob/master/README.md#github-actions or should we put it into a separate markdown file? Other ideas?

[casey]

I think we should move the full yaml file into contrib/github-actions-install-just.yaml, and link to it in the #github-actions section of the readme. Since it's probably faster to install just using a package manager that's already present on the actions runners, I think we should link to it at the beginning of the #github-actions section, before the two alternative methods.

[cclauss]

The file that CI/pages is complaining about will exist only after this pull request is merged.

[cclauss]

We can add the link from README.md in a separate PR to placate CI/pages.

[casey]

Okay, sorry to waffle, but looking at the workflow, I think it can be made short enough that we should just stick it in the readme, which also means we don't have to worry about a broken link.

By avoiding using a matrix, we can boil it down to:

name: install-just
on: [pull_request, push]
jobs:
  windows:
    runs-on: windows-latest
    steps:
    - run: choco install just
  macos:
    runs-on: macos-latest
    steps:
    - run: brew install just
  ubuntu:
    runs-on: ubuntu-latest
    steps:
    - run: sudo snap install --edge --classic just

Which will be pretty easy for users to insert into their existing workflow. What do you think?

[cclauss]

The working script is 3 lines longer than the matrix script and does not set fast-fail to false but it is easier to understand and most visitors will only be interested to use the Ubuntu bit.

[casey]

Nice, looks good to me! Thank you!