[9.x] Update PackageManifest::$vendorPath initialisation for cases, w…

…hen composer vendor dir is not in project directory (#41463)

* Update PackageManifest.php

It's possible, that for some reason, vendor dir could be in another directory (https://getcomposer.org/doc/03-cli.md#composer-vendor-dir).
For example, when you change vendor dir in docker image for easier app bind mounting (https://github.com/BretFisher/php-docker-good-defaults/blob/master/Dockerfile).

* Update PackageManifest.php

* formatting

Co-authored-by: Taylor Otwell <taylor@laravel.com>

src/Illuminate/Foundation/PackageManifest.php

@@ -4,6 +4,7 @@
 
 use Exception;
 use Illuminate\Filesystem\Filesystem;
+use Illuminate\Support\Env;
 
 class PackageManifest
 {
@@ -55,7 +56,7 @@ public function __construct(Filesystem $files, $basePath, $manifestPath)
         $this->files = $files;
         $this->basePath = $basePath;
         $this->manifestPath = $manifestPath;
-        $this->vendorPath = $basePath.'/vendor';
+        $this->vendorPath = Env::get('COMPOSER_VENDOR_DIR') ?: $basePath.'/vendor';
     }
 
     /**