[cipd] Do not rewrite ERRORLEVEL

cipd.bat is called from bootstrap/win/win_tools.bat. ERRORLEVEL in
win_tools.bat script should not mean return code of cipd.bat.

Bug: none
Change-Id: Ia425d5054fa9c1e1f84b4a10decc265669911646
Reviewed-on: https://chromium-review.googlesource.com/987954Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>

cipd.bat

@@ -25,6 +25,6 @@ if not errorlevel 0 goto :END
 
 :END
 endlocal & (
-  set ERRORLEVEL=%ERRORLEVEL%
+  set EXPORT_ERRORLEVEL=%ERRORLEVEL%
 )
-exit /b %ERRORLEVEL%
+exit /b %EXPORT_ERRORLEVEL%