Fix infra_to_superproject for Linux

When using shell=True on Linux, subprocess only executes first item on
the list. In this case, it calls fetch without providing config. Windows
works just fine.

R=jojwang@google.com

Fix: 1433139
Change-Id: Ib3b0bef4d05c0ff076d936e1d8a00047158e2e96
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4428533
Reviewed-by: Joanna Wang <jojwang@chromium.org>
Commit-Queue: Joanna Wang <jojwang@chromium.org>
Auto-Submit: Josip Sokcevic <sokcevic@chromium.org>

infra_to_superproject.py

@@ -60,7 +60,8 @@ def main(argv):
     print('including internal code in checkout')
   else:
     cmds.append('infra')
-  fetch = subprocess.Popen(cmds, cwd=source, shell=True)
+  shell = sys.platform == 'win32'
+  fetch = subprocess.Popen(cmds, cwd=source, shell=shell)
   fetch.wait()