treewide: use /usr/bin/env where appropriate

Some Unix systems (GuixSD, NixOS) do not install programs like
Bash and Python to /usr/bin, and /usr/bin/env has to be used to
locate these instead.

Change-Id: I7546bcb881c532adc984577ecb0ee2ec4f2efe00
Signed-off-by: Yegor Timoshenko <yegortimoshenko@riseup.net>
Reviewed-on: https://review.coreboot.org/28953
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

util/scripts/get_maintainer.pl

@@ -1,4 +1,4 @@
-#!/usr/bin/perl -w
+#!/usr/bin/env perl
 # (c) 2007, Joe Perches <joe@perches.com>
 #           created from checkpatch.pl
 #
@@ -11,6 +11,7 @@
 # Licensed under the terms of the GNU GPL License version 2
 
 use strict;
+use warnings;
 
 my $P = $0;
 my $V = '0.26';