process: fix default env for process.execve

The `env` parameter for `process.execve` is documented to default
to `process.env`.

Author: richardlau

lib/internal/process/per_thread.js

@@ -279,7 +279,7 @@ function wrapProcessMethods(binding) {
     return true;
   }
 
-  function execve(execPath, args = [], env) {
+  function execve(execPath, args = [], env = process.env) {
     emitExperimentalWarning('process.execve');
 
     const { isMainThread } = require('internal/worker');