allow set callback and environment together

Example:
run "my_command", sub {
     my ($stdout, $stderr) = @_;
     my $server = Rex::get_current_connection()->{server};
     say "[$server] $stdout\n";
   },
   env => {
     env_var_1 => "the value for 1",
     env_var_2 => "the value for 2",
   };