Move to a different CI again?

[Minoru]

This project currently uses GitHub Actions, but its "cache" action turned out to be quite unreliable:

• on macOS, cached executables aren't executable after being restored. Our cache contains the compiler, so this is obviously a problem;
• Windows doesn't ship tar, so restoring the cache always fails. This is less catastrophic than the macOS one, since the job simply takes longer to finish (≈20 minutes versus ≈2 minutes).

What can I do about it?

1. be a good FLOSS citizen and fix the "cache" action. Yeah, no, I'm not that married to GitHub Actions.

2. disable the cache for macOS and Windows. That means waiting ≈20 minutes instead of ≈2 minutes. Don't like the wait.

3. move to a different CI provider. There's plenty of them out there, and that's what I'm inclined to do.

I'll decide after I slept on this.

[Minoru]

Actually, our problem with Windows is a bit different: the cache is simply empty, even though tar.exe doesn't fail (1, 2). I don't care enough to debug it though.

[Minoru]

With that said: let's try AppVeyor.

[Minoru]

AppVeyor's pain points:

• default images don't have Stack, so I had to install it. The for.matrix.only construct didn't work for install script;
• environment doesn't let me reference existing variables when settings new ones, so PATH: $HOME/.local/bin:$PATH does not work

I've sunk more time into this than intended, so I'm laying this aside for now. I'm not removing AppVeyor integration from the repo, so if you're interested in playing with this, you can submit a draft PR with .appveyor.yml and play with that.

[Minoru]

AppVeyor integration is annoying, since it's triggering builds even if no .appveyor.yml is present. I disabled it for now.

[Minoru]

aec45c9 moves us back to GitHub Actions, with just Linux for now. There are separate issues about Windows (#24) and macOS (#41), so I'm closing this one.