I found an article that explain the meaning of the new blocks-manifest.php file

This article

I guess there is presently a bug in the wordpress (or webpack) scripts, the npm start script remove blocks-manifest.php file from the build firectory.

In the meantime if you want to continue to work on your block until the bug is fixed, you can modify the main plugin file and register your blocks one by one in the old way like this:

function create_block_block_toto_block_init() {
    register_block_type( __DIR__ . '/build/toto1' );
    register_block_type( __DIR__ . '/build/toto2' );
}
add_action( 'init', 'create_block_block_toto_block_init' );

It will work, blocks-manifest.php will be ignored