[autoninja] Respect user-provided AUTONINJA_BUILD_ID on Windows

Follow-up on https://crrev.com/c/3569027

Change-Id: I4b7830c83939d8a75cde7cae9001e89cf68f0d79
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3575946
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Yannic Bonenberger <yannic.bonenberger@gmail.com>

autoninja.bat

@@ -7,8 +7,10 @@ setlocal
 
 set scriptdir=%~dp0
 
-:: Set unique build ID.
-FOR /f "usebackq tokens=*" %%a in (`%scriptdir%python-bin\python3.bat -c "import uuid; print(uuid.uuid4())"`) do set AUTONINJA_BUILD_ID=%%a
+if not defined AUTONINJA_BUILD_ID (
+  :: Set unique build ID.
+  FOR /f "usebackq tokens=*" %%a in (`%scriptdir%python-bin\python3.bat -c "import uuid; print(uuid.uuid4())"`) do set AUTONINJA_BUILD_ID=%%a
+)
 
 :: If a build performance summary has been requested then also set NINJA_STATUS
 :: to trigger more verbose status updates. In particular this makes it possible