Allow ATOM_HOME to be reassigned if the new value is valid

src/update-process-env.js

@@ -1,5 +1,6 @@
 /** @babel */
 
+import fs from 'fs'
 import {spawnSync} from 'child_process'
 
 const ENVIRONMENT_VARIABLES_TO_PRESERVE = new Set([
@@ -37,6 +38,10 @@ function updateProcessEnv (launchEnv) {
         process.env[key] = envToAssign[key]
       }
     }
+
+    if (envToAssign.ATOM_HOME && fs.existsSync(envToAssign.ATOM_HOME)) {
+      process.env.ATOM_HOME = envToAssign.ATOM_HOME
+    }
   }
 }