scripts: fix buildstats diff/summary hard bound to host python3

Somehow these two got left behind and hence on older hosts that
are using buildtools for a newer python - they will still fail.

(From OE-Core rev: db3f62378c54ebdfbf5cccdba4c83f503372138f)

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit f1b11b83a6dd991f393940741c930691463e6d53)
Signed-off-by: Steve Sakoman <steve@sakoman.com>

scripts/buildstats-diff

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 #
 # Script for comparing buildstats from two different builds
 #

scripts/buildstats-summary

@@ -1,4 +1,4 @@
-#! /usr/bin/python3
+#!/usr/bin/env python3
 #
 # Dump a summary of the specified buildstats to the terminal, filtering and
 # sorting by walltime.