whoami: factor conditional

* Make code even simpler by splitting USER and USERNAME checks into their own subsequent closures
* Tested this by commenting the other subs and running 'perl env -i USERNAME=bob perl whoami'

bin/whoami

@@ -26,7 +26,8 @@ my @coderefs = (
 	sub { getpwuid($>) },
 	sub { require Win32; Win32::LoginName() },
 	sub { getlogin },
-	sub { defined $ENV{USER} ? $ENV{USER} : $ENV{USERNAME} },
+	sub { $ENV{'USER'} },
+	sub { $ENV{'USERNAME'} },
 	);
 
 foreach my $coderef ( @coderefs ) {