Keep `civicrm-version.php` up-to-date without running GenCode on all builds

We aim to allow installing CiviCRM on Drupal 8 via composer/git without requiring the admin to run GenCode.

The file civicrm-version.php is generated by GenCode. A typical copy looks like this:

<?php
/**
 * Get the CiviCRM version.
 */
function civicrmVersion() {
  return array(
    'version' => '4.7.30',
    'cms' => 'Drupal',
    'revision' => '4.7.30',
  );
}

A simple fix might be:

• Commit the file to civicrm-core.git (much like in #1 (closed)).
• Update tools/bin/scripts/set-version.php to maintain this file. (That script is part of the workflow whenever setup a new release.)

There's one catch: the cms field varies depending on the local runtime. However, we may be lucky here -- in a cursory check, I only found a couple places that relied on it. The task here would be to take a closer look and potentially remove this element. We'd also need to resolve these stale references to cms:

• civicrm.drush.inc (in civicrm-backdrop-1.x, civicrm-drupal-6.x, civicrm-drupal-7.x, civicrm-drupal-8.x)
  • Use constants or functions from Drupal/Drush/Backdrop to detect the active environment.
• install/index.php and install/template.html
  • Remove or rewrite any optional sanity checks that rely on this field.
  • Use $installType to backfill $civicrm_version['cms'] (like the WordPress conditional does)