Well, looks like I have found the answer. The action should be changed like so:
$action = 'install-plugin';
$slug = 'akismet';
wp_nonce_url(
add_query_arg(
array(
'action' => $action,
'plugin' => $slug
),
admin_url( 'update.php' )
),
$action.'_'.$slug
);
The answer was found in line 275 in the source code: https://core.trac.wordpress.org/browser/tags/3.9.1/src/wp-admin/includes/plugin-install.php