<?php
$args = array(
    'meta_query' => array(
        'relation' => 'OR',
        array(
            'key' => 'skill',
            'value' => 'php',
            'compare' => '='
        ),

        array(
            'key' => 'skill',
            'value' => 'nodejs',
            'compare' => '='
        )
    )
);
$user_query = new WP_User_Query( $args );