Run builds only once on pull requests

Pushing a branch and creating a pull request would run all the builds twice with the previous formulation.

Trying the yml from here: https://github.community/t/how-to-trigger-an-action-on-push-or-pull-request-but-not-both/16662/3

.github/workflows/build.yml

 name: Build
 
-on: [push, pull_request]
-
+on:
+  push:
+    branches:
+      - master
+  pull_request:
 jobs:
 
   LinuxCuda: