bot_update: fix the check for is running on linux

Since python3.3, the platform value has been changed from linux2 or
linux3 to linux. The bug has stopped bot_update script from emitting
proces tree periodically that will be helpful when debugging.

R=sokcevic

Bug: 1455926
Change-Id: Id6ab4ddf46eaa322a8b5470baa7ba6cc656a6bb3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4654290
Auto-Submit: Yiwei Zhang <yiwzhang@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@chromium.org>
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Commit-Queue: Yiwei Zhang <yiwzhang@google.com>

recipes/recipe_modules/bot_update/resources/bot_update.py

@@ -158,7 +158,7 @@ class RepeatingTimer(threading.Thread):
 
 def _print_pstree():
   """Debugging function used to print "ps auxwwf" for stuck processes."""
-  if sys.platform.startswith('linux2'):
+  if sys.platform.startswith('linux'):
     # Add new line for cleaner output
     print()
     subprocess.call(['ps', 'auxwwf'])