.gitlab-ci.yml: replace build matrix by independant build entries

.gitlab-ci.yml

@@ -1,16 +1,19 @@
-image: ubuntu:focal
-
 before_script:
   - apt-get update
-  - apt-get -y install ${JDK_PACKAGE}
-  - apt-get -y install gradle
 
 after_script:
   - gradle --version
 
-build:
-  parallel:
-    matrix:
-      - JDK_PACKAGE: [ openjdk-8-jdk, openjdk-11-jdk ]
+focal-jdk8:
+  image: ubuntu:focal
   script:
+    - apt-get -y install openjdk-8-jdk
+    - apt-get -y install gradle
+    - gradle build --stacktrace
+
+focal-jdk11:
+  image: ubuntu:focal
+  script:
+    - apt-get -y install openjdk-11-jdk
+    - apt-get -y install gradle
     - gradle build --stacktrace