Option to use an environment variable for the Craft license key

[MoritzLost]

We use an environment variable CRAFT_LICENSE_KEY to set the Craft license. This avoids some problems with local or staging environments, and makes it easier to buy and install a new license on a production server. We use this in our bootstrap.php:

if ($license = App::env('CRAFT_LICENSE_KEY')) {
    define('CRAFT_LICENSE_KEY', $license);
}

This works and is recognized by Craft. But Craft doesn't know that the license comes from an environment variable. This means that when Craft checks for updates, it will always write the new license key to config/license.key. We then have to copy and paste that to our .env file and then delete the license.key file.

What I'd like to see is for Craft to write the license key to our .env file automatically, similar to plugin license keys, when those are defined as environment variables. For that to work, Craft would need to know which environment variable is used for the license key. This could either be hardcoded (e.g. to CRAFT_LICENSE_KEY) or made a config setting. In either case, Craft should also check this environment variable automatically before attempting to read the config/license.key file. Then we could even drop the snippet above from our bootstrap.php. This new behavior could also be made opt-in, e.g. through a general setting useEnvironmentVariableForLicenseKey, with allowed values either true or false, or maybe the name of the environment variable to use as a string?