composer: importare repository locali tramite mirroring

[astros3]

+ cat composer.json
{
    "repositories": {
        "pluto/pluto": {
            "type": "path",
            "url": "/mnt/repo/pluto"
        }
    }
}

+ cat /mnt/repo/pluto/composer.json
{
    "name": "pluto/pluto",
    "version": "1"
}

+ COMPOSER_MIRROR_PATH_REPOS=1 composer require pluto/pluto
Using version ^1.0 for pluto/pluto
./composer.json has been updated
Loading composer repositories with package information
Updating dependencies (including require-dev)
  - Installing pluto/pluto (1.0.0)
    Symlinked from pluto/

Ma perchè il repository, invece di venire copiato (mirroring), viene creato il collegamento simbolico?
Eppure ho specificato la variabile d'ambiente COMPOSER_MIRROR_PATH_REPOS=1.