benchmarks comparer: Use /usr/bin/env to find perl

This is needed on NixOS. FHS does say if perl is installed it shoul be
found at `/usr/bin/perl`, so can't say this is more portable in general,
but doesn't hurt FHS systems either

Author: Ericson2314

benchmarks/bench-cmp.pl

@@ -1,4 +1,4 @@
-#!/usr/bin/perl
+#!/usr/bin/env perl
 use warnings;
 use strict;