Add support for GitHub Actions

[olafurpg]

GitHub Actions (https://github.com/features/actions) offer a promising alternative to Travis CI. This PR sets up automatic CI releases for this build via GitHub Actions so that we can disable the Travis CI.

[alexarchambault]

@olafurpg Is it an sbt launcher generated by coursier bootstrap? Until the install command of the coursier CLI is stable, I'd recommend using the customized sbt-extras scripts instead of committing a binary launcher. It downloads the latest version of the sbt-launcher itself (I mean, the launcher version is hard-coded in the script, but the script itself gets automatically updated).

[olafurpg]

@alexarchambault thank you for the suggestion! The olafurpg/setup-java custom action new downloads the sbt-extras script from master on every job https://github.com/olafurpg/setup-java/blob/5fbc7abe97954a5348df0397160d921ad65d2bd6/src/install.ts#L36

The coursier sbt-launcher seems to fail on sbt 1.3.x with

[error] java.lang.NoClassDefFoundError: lmcoursier/CoursierConfiguration

Any idea if it's possible to use the launcher with sbt 1.3?

[olafurpg]

GitHub Actions is looking super promising so far. It's a joy to write custom actions and the lack of caching doesn't seem to be a blocker, clean installing OpenJDK + launching sbt + compiling a bit of Scala code seems to complete in around one minute!

The code for the custom action to install JDK + sbt is here https://github.com/olafurpg/setup-java/blob/5fbc7abe97954a5348df0397160d921ad65d2bd6/src/install.ts#L36 I think it might even work on Windows, although I haven't tested that yet.

[alexarchambault]

@olafurpg I would blame the too old sbt-coursier plugin you're relying on. But you can remove sbt-coursier altogether with sbt 1.3.x. That's unless you're using sbt-shading though, that is pulled in project/plugins.sbt here, but is unused apparently.

[olafurpg]

Thanks a lot for the help @alexarchambault ! I ended up installing the regular sbt-extras as sbt and the coursier-based one as csbt so people can choose which one they want. Let's give this a spin 💃

[alexarchambault]

@olafurpg You may want to add that line to remain compatible with pre-1.3 sbt versions. (Was reminded that by @eed3si9n in sbt-coursier…)