Turn off creds caching for windows devs.

Windows devs use luci-auth as the auth mechanism (as credshelper).  By
default, credshelper-based auth is cached, but that doesn't work quite
correctly with luci-auth (it does it's own caching).  It can cause
intermittent auth failures during a build, which can slow down build
performance.

Bug: b/335525655
Change-Id: Ic4ef25cfa8e0f37929170e306e8bf57e737a4a72
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5473742
Reviewed-by: Ben Segall <bentekkie@google.com>
Auto-Submit: Michael Savigny <msavigny@google.com>
Commit-Queue: Ben Segall <bentekkie@google.com>

reclient_helper.py

@@ -284,6 +284,9 @@ def set_win_defaults():
     # Set execution strategy to remote_local_fallback while racing performance
     # on windows is addressed.
     os.environ.setdefault("RBE_exec_strategy", "remote_local_fallback")
+    # Turn off creds caching for windows, as luci-auth as credshelper shouldn't
+    # use it.
+    os.environ.setdefault("RBE_enable_creds_cache", "false")
 
 
 def workspace_is_cog():