crosperf+cros_utils: Swap to #!/usr/bin/env.

`cros lint` was recently updated to complain about
using #!/usr/bin/python{,2,3} directly. Instead, it prefers
/usr/bin/env python{2,3} now.

BUG=None
TEST=./run_tests.sh passes; linter seems much happier with crosperf.

Change-Id: I9b88a7af1f8e03b7c870781c3d8dc1b872a3dfc3
Reviewed-on: https://chrome-internal-review.googlesource.com/286341
Commit-Ready: Luis Lozano <llozano@chromium.org>
Tested-by: Luis Lozano <llozano@chromium.org>
Reviewed-by: Luis Lozano <llozano@chromium.org>
diff --git a/crosperf/results_cache_unittest.py b/crosperf/results_cache_unittest.py
index 85ed8a8..5a824b1 100755
--- a/crosperf/results_cache_unittest.py
+++ b/crosperf/results_cache_unittest.py
@@ -1,4 +1,4 @@
-#!/usr/bin/python2
+#!/usr/bin/env python2
 
 # Copyright (c) 2011 The Chromium OS Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be