Fix the installer when using "other"

[byrond]

drupal.root is not set when selecting the "other" option during the-build installation. This results in the following error:

install > setup-web-root:

     [echo] Moving the docroot/ directory to ${drupal.root}/
sh: 1: Bad substitution
[phingcall] /var/www/html/vendor/palantirnet/the-build/targets/install.xml:137:12: /var/www/html/vendor/palantirnet/the-build/targets/install.xml:145:125: Task exited with code 2

BUILD FAILED
/var/www/html/vendor/bin/../palantirnet/the-build/targets/install.xml:86:44: Execution of the target buildfile failed. Aborting.

• Defines a default value for drupal.root during installation. The switch structure could be simplified, but this method leaves room for other properties to be set for each environments if needed.
• Modifies the setup-web-root target to check for a core subdirectory, because DDEV will create whatever it is configured to use as its docroot if it doesn't already exist, and it will add settings.ddev.php there. Also, we remove the new docroot before moving the old one.

Testing

• composer create-project palantirnet/drupal-skeleton example dev-develop --no-interaction
• cd example
• composer require --dev palantirnet/the-build:dev-fix-installer-for-other
• Follow the instructions in https://github.com/palantirnet/drupal-skeleton#steps (starting at step 2).
• Choose "other" during the-build-installer.

[becw]

I made one minor change. Other than that, 👍