Ignore workflow runs on master branch

This master branch is just an alias to the main branch for legacy
reasons, so there's no need to run workflows on both branches.

Author: wpaulino

.github/workflows/build.yml

@@ -1,6 +1,12 @@
 name: Continuous Integration Checks
 
-on: [push, pull_request]
+on:
+  push:
+    branches-ignore:
+      - master
+  pull_request:
+    branches-ignore:
+      - master
 
 jobs:
   build: