scripts/oe-build-perf-test: Use python3 from the environment

On test machines we have python3 available at alternative locations. Use
these rather from the evnrionment rather than a hardcoded path.

(From OE-Core rev: 81ee68f39a155c79443d45321ce4b81d9bf6675e)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/oe-build-perf-test

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 #
 # Build performance test script
 #