Conditionally attach a library in Drupal based on environment

THEMENAME.libraries.yml

my_lib:
  js:
    https://path-to-file/my-script.min.js: {type: external, minified: true}

my_lib.test:
  js:
    https://path-to-file/my-script-test.min.js: {type: external, minified: true}
    
my_lib.dev:
  js:
    https://path-to-file/my-script-dev.js: {type: external}

THEMENAME.theme

<?php

// Replace THEMENAME with the name of the theme or module where this code will be added.

/**
 * Implements hook_page_attachments_alter().
 */
function THEMENAME_page_attachments_alter(array &$page) {
  // Default to the Dev library.
  $lib_name = 'THEMENAME/my_lib.dev';

  // Check if this is Pantheon.
  if (isset($_ENV['PANTHEON_ENVIRONMENT'])) {
    // Change the library if this is the Live or Test site.
    switch ($_ENV['PANTHEON_ENVIRONMENT']) {
      case 'live':
        $lib_name = 'THEMENAME/my_lib';
        break;
      case 'test':
        $lib_name = 'THEMENAME/my_lib.test';
        break;
    }
  }

  // Attach the library to the page.
  $page['#attached']['library'][] = $lib_name;
}