set path to mingw on kokoro

tools/internal_ci/windows/grpc_build_artifacts.bat

@@ -34,10 +34,13 @@ rename C:\Python34_32bit Python34_32bits
 rename C:\Python35_32bit Python35_32bits
 rename C:\Python36_32bit Python36_32bits
 
+pacman -S --noconfirm mingw64/mingw-w64-x86_64-gcc mingw32/mingw-w64-i686-gcc
+
 @rem make sure msys binaries are preferred over cygwin binaries
 @rem set path to python 2.7
 set PATH=C:\tools\msys64\usr\bin;C:\Python27;%PATH%
 
+
 @rem enter repo root
 cd /d %~dp0\..\..\..
 

tools/run_tests/artifacts/build_artifact_python.bat

@@ -27,8 +27,8 @@
 @rem (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 @rem OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-
-set PATH=C:\%1;C:\%1\scripts;C:\msys64\mingw%2\bin;%PATH%
+@rem set path to python & mingw compiler
+set PATH=C:\%1;C:\%1\scripts;C:\msys64\mingw%2\bin;C:\tools\msys64\mingw%2\bin;%PATH%
 
 pip install --upgrade six
 pip install --upgrade setuptools