Add stages in exercise 1 in episode 6 - optimizing CI pipeline

* Use stages instead of a pure stageless pipeline while still using the
needs keyword to reorder execution of CI jobs.

.gitlab-ci.yml

@@ -5,6 +5,12 @@
 
 ---
 
+stages:
+  - lint
+  - build
+  - test
+  - run
+
 default:
   image: python:3.10
 
@@ -19,6 +25,7 @@ default:
       - VERSION: [ "10", "11", "12" ]
 
 license_compliance:
+  stage: lint
   before_script:
     - pip install --upgrade pip
     - pip install reuse
@@ -27,6 +34,7 @@ license_compliance:
   interruptible: true
 
 lint:
+  stage: lint
   before_script:
     - pip install --upgrade pip
     - pip install cpplint
@@ -38,6 +46,7 @@ build:gcc:
   extends:
     - .install_dependencies
     - .compiler_versions
+  stage: build
   image: gcc:${VERSION}
   script:
     - cmake -S . -B build_gcc_${VERSION}
@@ -51,6 +60,7 @@ test:gcc:
   extends:
     - .install_dependencies
     - .compiler_versions
+  stage: test
   image: gcc:${VERSION}
   script:
     - cd build_gcc_${VERSION} && ctest
@@ -60,6 +70,7 @@ test:gcc:
   interruptible: true
 
 run:
+  stage: run
   image: gcc:11
   script:
     - ./build_gcc_11/bin/helloWorld