Enable to set $ENV_DEBUG package locally

Now, We can set envar name by parameter in use line.

    use Log::Minimal env_debug => 'FOO_DEBUG';
    
    $ENV{LM_DEBUG}  = 1;
    $ENV{FOO_DEBUG} = 0;
    debugf("hello"); # no output
    
    $ENV{FOO_DEBUG} = 1;
    debugf("world"); # print message