Add test build workflow for PRs (#113)

Author: ManlyMarco

.github/workflows/build.yml

@@ -1,6 +1,6 @@
 name: Build
 
-on: [push]
+on: [push, pull_request]
 
 jobs:
   test:
@@ -18,6 +18,7 @@ jobs:
       run: ./build.ps1 --target=Test
   build:
     runs-on: windows-latest
+    if: github.ref == 'refs/heads/master'
     needs: test
 
     steps: