This adds the setup-gradle action; which has the following improvements:
* Improved cacheing (compared to setup-java's Gradle cache)
* Validates the gradle wrapper jar
This switches our builds over to GitHub Actions!
TODO:
* Add macOS/amd64 native-image builds; this isn't working right now
* Patch musl with security patches
* Add benchmark jobs over time
As part of this build, PRBs will now only run `./gradlew check` on Linux,
but other jobs can be run using slash commands, e.g. `[windows]`
to run `./gradle check` on Windows.
