CHG: Run pipeline "build" only on master

.gitlab-ci.yml

@@ -13,16 +13,21 @@ variables:
 cache:
   paths:
     - .cache/pip
-    - venv/
 
 before_script:
   - python -V  # Print out python version for debugging
   - pip install pep517
 
+stages:
+  - build
+
 build:
+  stage: build
   script:
     - python -m pep517.build .
   artifacts:
     paths:
       - dist/*.whl
     expire_in: 2 days
+  only:
+    - master