Use python to run retrace

Just running the script does not work on windows

Change-Id: I2b07fc91634136712c1b4aa0a53b0416b28df5a5
diff --git a/build.gradle b/build.gradle
index 6afc31a..b6b1772 100644
--- a/build.gradle
+++ b/build.gradle
@@ -1508,7 +1508,7 @@
     if (project.hasProperty('r8lib') || project.hasProperty('r8lib_no_deps')) {
         def out = new StringBuffer()
         def err = new StringBuffer()
-        def command = "tools/retrace.py"
+        def command = "python tools/retrace.py"
         def header = "RETRACED STACKTRACE";
         if (System.getenv('BUILDBOT_BUILDERNAME') != null
                 && !System.getenv('BUILDBOT_BUILDERNAME').endsWith("_release")) {