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.

.gitattributes

@@ -1,5 +1,6 @@
 # linguist-generated would suppress files in diffs
 **/src/test/files/** linguist-vendored
+.github/workflows/* linguist-generated
 
 /docs/** linguist-documentation