How to show metabox on page if it is using a template

Use the page-specific meta box hook & accept the $post object that’s passed to it – then you can check if it currently has a page template of sky-template.php.

add_action( 'add_meta_boxes_page', function ( $post ) {
    if ( $post->_wp_page_template === 'sky-template.php' ) {
        add_meta_box( 'sky_page_excerpt', 'SkyScraper Page Excerpt and Links', 'sky_page_excerpts', 'page', 'advanced', 'high' );
    }
});

Note: this will only work on submit/refresh i.e. when you first (de)select the template when editing, you’ll need to save changes and have the page reload for the meta box to take affect.