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/BuildNativeJob.pkl

@@ -0,0 +1,47 @@
+extends "GradleJob.pkl"
+
+import "@gha/actions/Artifact.pkl"
+
+/// Whether to link to musl. Otherwise, links to glibc.
+musl: Boolean(implies(module.os == "linux")) = false
+
+/// The Gradle project under which to generate the executable
+project: String
+
+extraGradleArgs {
+  when (os == "macOS" && arch == "amd64") {
+    "-Dpkl.targetArch=\(module.arch)"
+  }
+  when (musl) {
+    "-Dpkl.musl=true"
+  }
+}
+
+
+steps {
+  when (musl) {
+    new {
+      name = "Install musl and zlib"
+      run = read("../scripts/install_musl.sh").text
+    }
+  }
+  new {
+    name = "gradle buildNative"
+    shell = "bash"
+    run =
+      """
+      ./gradlew \(module.gradleArgs) \(project):buildNative
+      """
+  }
+  new Artifact.Upload {
+    name = "Upload executable artifacts"
+    with {
+      name =
+        if (musl)
+          "executable-\(project)-alpine-\(module.os)-\(module.arch)"
+        else
+          "executable-\(project)-\(module.os)-\(module.arch)"
+      path = "\(project)/build/executable/**/*"
+    }
+  }
+}