enable stack dumping on appveyor

appveyor.yml

@@ -30,6 +30,7 @@ environment:
   GIT_CACHE_PATH: C:\Users\electron\libcc_cache
   DISABLE_CRASH_REPORTER_TESTS: true
   ELECTRON_OUT_DIR: Default
+  ELECTRON_ENABLE_STACK_DUMPING: 1
 build_script:
   - ps: >-
       if(($env:APPVEYOR_PULL_REQUEST_HEAD_REPO_NAME -split "/")[0] -eq ($env:APPVEYOR_REPO_NAME -split "/")[0]) {