[newauth] Enable by default (reland) (skip bots)

Originally
https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6418563

Bug: 404613530
Bug: 408427309
Change-Id: I17bf758f2dd342493d42c1b2dd233696f3ebef47
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6440137
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Allen Li <ayatane@chromium.org>

newauth.py

@@ -35,7 +35,9 @@ def SwitchedOn() -> bool:
 
 def Default() -> bool:
     "Returns default enablement status for new auth stack."
-    return False
+    if os.getenv('SWARMING_BOT_ID'):
+        return False
+    return True
 
 
 def _HasGitcookies() -> bool: