use more portable /usr/bin/env perl for scripts

This is useful, e.g. if your perl is not installed in `/usr/bin` or you
want to give a specific perl precendence over the one installed in
`/usr/bin`. The `-w` is not needed since there is also `use warnings;`
in these scripts.

qualfa2fq.pl

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

xa2multi.pl

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