autoninja: run ninjalog_uploader in new session

Bug: 390332232
Change-Id: I5bbe95e1d26899ab7dfbba418d44e250d9277fc9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6171585
Reviewed-by: Junji Watanabe <jwata@google.com>
Commit-Queue: Fumitoshi Ukai <ukai@google.com>
Auto-Submit: Fumitoshi Ukai <ukai@google.com>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>

autoninja.py

@@ -478,6 +478,7 @@ def _upload_ninjalog(args, exit_code, build_duration):
         cmd,
         stdout=subprocess.DEVNULL,
         stderr=subprocess.DEVNULL,
+        start_new_session=True,
         creationflags=creationflags,
     )