Pass PYTHON env var to memory tracker

It was not honoring PYTHON3_VERSION when running
with GLOBAL_VENV=false, this patch fixes it.

Related-Bug: #2109591
Change-Id: Ib34c099b897e59fce24cab6e98dc31a505e4922e

Author: karelyatin

lib/dstat

@@ -33,7 +33,7 @@ function start_dstat {
     # To enable memory_tracker add:
     #    enable_service memory_tracker
     # to your localrc
-    run_process memory_tracker "$TOP_DIR/tools/memory_tracker.sh" "" "root"
+    run_process memory_tracker "$TOP_DIR/tools/memory_tracker.sh" "" "root" "PYTHON=python${PYTHON3_VERSION}"
 
     # TODO(jh): Fail when using the old service name otherwise consumers might
     # never notice that is has been removed.