fix fetching of GCE default credentials on kokoro windows

tools/internal_ci/helper_scripts/prepare_build_windows.bat

@@ -18,6 +18,14 @@ set PATH=C:\tools\msys64\usr\bin;C:\Python27;%PATH%
 
 bash tools/internal_ci/helper_scripts/gen_report_index.sh
 
+@rem Add GCE DNS server and disable IPv6 to:
+@rem 1. allow resolving metadata.google.internal hostname
+@rem 2. make fetching default GCE credential by oauth2client work
+netsh interface ipv4 add dnsservers "Local Area Connection 8" 10.240.0.1 index=1
+netsh interface teredo set state disabled
+netsh interface 6to4 set state disabled
+netsh interface isatap set state disabled
+
 @rem Needed for big_query_utils
 python -m pip install google-api-python-client