Add pull request trigger on workflow

Author: simonoakesepimorphics

.github/workflows/build.yaml

@@ -1,7 +1,7 @@
 # This workflow will build a Java project with Maven
 # For more information see: https://help.github.com/actions/language-and-framework-guides/building-and-testing-java-with-maven
 name: Java CI with Maven
-on: push
+on: [push, pull_request]
 jobs:
   build:
     runs-on: ubuntu-latest