removing pull_request builds from workflow

.github/workflows/build.yml

@@ -1,4 +1,4 @@
-on: [ push, pull_request ]
+on: [ push ]
 
 jobs:
   build_ubuntu:
@@ -105,4 +105,3 @@ jobs:
     - name: msbuild
       run: |
         msbuild windows\ZeroTierOne.sln /m /p:Configuration=Release  /property:Platform=x64 /t:ZeroTierOne:Rebuild        
-