Fix logfile output getting stomped

The LOGFILE would get stomped due to a non-appending tee command.

Change-Id: I851ee83171c27fc425c0d666fcf84437f0d118b2
diff --git a/stack.sh b/stack.sh
index 545c071..4dfa9aa 100755
--- a/stack.sh
+++ b/stack.sh
@@ -2287,7 +2287,7 @@
 if [[ -n "$LOGFILE" ]]; then
     exec 1>&3
     # Force all output to stdout and logs now
-    exec 1> >( tee "${LOGFILE}" ) 2>&1
+    exec 1> >( tee -a "${LOGFILE}" ) 2>&1
 else
     # Force all output to stdout now
     exec 1>&3