Yes, the blocks can not be edited as HTML simply.
Try this:
1. When you add the link, select nofollow.
2. Add a CSS class to the button block, eg. social-link, so we can target the block in the PHP of step 3.
https://wordpress.com/support/wordpress-editor/adding-additional-css-classes-to-blocks/
3. Add this PHP snippet to replace the nofollow with me:
add_filter( 'render_block', function( $block_content, $block ) {
if ( ! empty( $block['attrs']['className'] ) && strpos( $block['attrs']['className'], 'social-link' ) !== false ) {
$block_content = str_replace('nofollow','me', $block_content );
}
return $block_content ;
}, 10, 2 );