[SKIP CI] Parallelize parts of PR build check workflow

.github/workflows/build_pull_request.yml

@@ -15,8 +15,8 @@ jobs:
       - name: Validate Gradle Wrapper
         uses: gradle/wrapper-validation-action@v1
 
-  build:
-    name: Build extension repo
+  build_multisrc_extensions:
+    name: Build multi-source extensions
     needs: check_wrapper
     if: "!startsWith(github.event.head_commit.message, '[SKIP CI]')"
     runs-on: ubuntu-latest
@@ -51,6 +51,26 @@ jobs:
           dependencies-cache-enabled: true
           configuration-cache-enabled: true
 
+  build_regular_extensions:
+    name: Build regular extensions
+    needs: check_wrapper
+    if: "!startsWith(github.event.head_commit.message, '[SKIP CI]')"
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout PR
+        uses: actions/checkout@v2
+
+      - name: Set up JDK 1.8
+        uses: actions/setup-java@v1
+        with:
+          java-version: 1.8
+
+      - name: Copy CI gradle.properties
+        run: |
+          mkdir -p ~/.gradle
+          cp .github/runner-files/ci-gradle.properties ~/.gradle/gradle.properties
+
       - name: Build extensions
         uses: eskatos/gradle-command-action@v1
         with: