Kokoro (Windows): Switch to python3

SPIRV-Tools now requires python3.

Bug: b/135609394
Change-Id: Ib33ddb57b0bdfa87abed94de6dbfb91eadbc31bf
diff --git a/tests/kokoro/gcp_windows/continuous.bat b/tests/kokoro/gcp_windows/continuous.bat
index 5334068..c8b1f1c 100644
--- a/tests/kokoro/gcp_windows/continuous.bat
+++ b/tests/kokoro/gcp_windows/continuous.bat
@@ -2,7 +2,7 @@
 
 SETLOCAL ENABLEDELAYEDEXPANSION
 
-SET PATH=%PATH%;C:\python27;C:\Program Files\cmake\bin
+SET PATH=C:\python36;C:\Program Files\cmake\bin;%PATH%
 set SRC=%cd%\git\SwiftShader
 
 cd %SRC%