workflows: build pull requests and master

With this change pull requests from forks are also built.
We yet need to verify if tags/releases are built or if we need to
specify the tags as well.

.github/workflows/build_tests.yml

@@ -1,6 +1,12 @@
 name: Build Tests
 
-on: [push]
+on:
+  push:
+    branches:
+    - 'master'
+  pull_request:
+    branches:
+    - '*'
 
 jobs:
   build: