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/PklProject

@@ -0,0 +1,10 @@
+amends "pkl:Project"
+
+dependencies {
+  ["pkl.impl.ghactions"] {
+    uri = "package://pkg.pkl-lang.org/pkl-project-commons/pkl.impl.ghactions@0.6.1"
+  }
+  ["gha"] {
+    uri = "package://pkg.pkl-lang.org/github.com/stefma/pkl-gha/com.github.action@0.0.6"
+  }
+}