Fix composer install with --no-cache flag

[justafish]

Installing drainpipe doesn't work with composer install --no-cache as calling $this->cache->getRoot() on a disabled cache returns /dev/null, resulting in the following error:

🪠 [Drainpipe] vendor/lullabot/drainpipe/scaffold/env/dotenv.php' missing from autoload-dev files

In Filesystem.php line 260:
                                                                               
  /dev/null/files/lullabot/drainpipe/bin/task/3.28.0 does not exist and could  
   not be created:                                                             

Which can be seen in the test added in the first commit: https://github.com/Lullabot/drainpipe/actions/runs/8647099796/job/23707817952

[leonel-lullabot]

I tested this solution on the project where I saw the error, and now I can run composer install --no-cache without errors. Thanks @justafish!