CI: add/update PR build action

Author: karlduderstadt

.github/workflows/build-pr.yml

@@ -0,0 +1,23 @@
+name: build PR
+
+on:
+  pull_request:
+    branches:
+      - master
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v2
+      - name: Set up Java
+        uses: actions/setup-java@v2
+        with:
+          java-version: '8'
+          distribution: 'zulu'
+          cache: 'maven'
+      - name: Set up CI environment
+        run: .github/setup.sh
+      - name: Execute the build
+        run: .github/build.sh