Switch to GitHub Actions (#1315)

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.

.github/jobs/SimpleGradleJob.pkl

@@ -0,0 +1,18 @@
+extends "GradleJob.pkl"
+
+name: String = command
+
+command: String
+
+os = "linux"
+
+steps {
+  new {
+    name = module.name
+    shell = "bash"
+    run =
+      """
+      ./gradlew \(module.gradleArgs) \(module.command)
+      """
+  }
+}