reduce nb CI builds (#1078)

.github/workflows/ci-testing.yml

@@ -1,6 +1,15 @@
 name: CI testing
 
-on: [push, pull_request]
+# https://help.github.com/en/actions/reference/events-that-trigger-workflows
+on:
+  # Trigger the workflow on push or pull request,
+  # but only for the master branch
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
 
 jobs:
   build: