Make it more likely that running the createArtTests gradle task will work on Windows.

I was a bit fast in landing the last change. It seems better to use
python as the executable with the script as the argument. I find
it more likely that that will work on Windows.

Change-Id: Ic380ba1efcf806f9523944773304a82e21c33c6d
diff --git a/build.gradle b/build.gradle
index 37745576..d33a127 100644
--- a/build.gradle
+++ b/build.gradle
@@ -300,7 +300,7 @@
     inputs.file createArtTestsScript
     outputs.dir outputDir
     dependsOn downloadDeps
-    commandLine "${createArtTestsScript}"
+    commandLine "python", createArtTestsScript
     workingDir = projectDir
 }