Use child_process.spawn instead of execFile

- Fixes #13084
- Kill child process on process exit, if it is running
- Ensure env is set correctly when new windows are launched

src/main-process/atom-application.coffee

@@ -509,7 +509,7 @@ class AtomApplication
   openPaths: ({initialPaths, pathsToOpen, executedFrom, pidToKillWhenClosed, newWindow, devMode, safeMode, windowDimensions, profileStartup, window, clearWindowState, addToLastWindow, env}={}) ->
     if not pathsToOpen? or pathsToOpen.length is 0
       return
-
+    env = process.env unless env?
     devMode = Boolean(devMode)
     safeMode = Boolean(safeMode)
     clearWindowState = Boolean(clearWindowState)