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/workflows/test_report.yml

@@ -0,0 +1,36 @@
+# Do not modify!
+# This file was generated from a template using https://github.com/StefMa/pkl-gha
+
+name: PR Test Reports
+'on':
+  workflow_run:
+    types:
+    - completed
+    workflows:
+    - Pull Request
+permissions:
+  contents: read
+jobs:
+  test-results:
+    name: Test Results
+    if: github.event.workflow_run.conclusion == 'success' || github.event.workflow_run.conclusion == 'failure'
+    permissions:
+      actions: read
+      checks: write
+    runs-on: ubuntu-latest
+    steps:
+    - name: Download artifacts
+      uses: dawidd6/action-download-artifact@v11
+      with:
+        path: artifacts
+        name: test-results-.*
+        name_is_regex: true
+        run_id: ${{ github.event.workflow_run.id }}
+    - name: Publish test results
+      uses: EnricoMi/publish-unit-test-result-action@v2
+      with:
+        commit: ${{ github.event.workflow_run.head_sha }}
+        comment_mode: 'off'
+        files: artifacts/**/*.xml
+        event_file: artifacts/test-results-event-file/event.json
+        event_name: ${{ github.event.workflow_run.event }}