Run VS build on PRs and each push

.github/workflows/vs.yml

@@ -1,15 +1,12 @@
 name: Visual Studio Build
 
-on:
-  workflow_dispatch:
+on: [push, pull_request]
 
 jobs:
   yosys-vcxsrc:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
-        with:
-          ref: 'master'
       - name: Cache sources
         id: cache-sources
         uses: actions/cache@v2