Preserve file permission when installing using composer project

[hansfn]

(This is a repost of geerlingguy/drupal-vm#1817 as requested by geerlingguy.)

Composer project ensures that the files directory (and maybe the sync directory when my PR gets approved) has correct permission. However, the build-composer-project task uses cp -r which doesn't preserve permission. Since umask is 022 it kills the writable bits. The easy fix is to add -p to the cp command.

PS! Windows users that uses the default vagrant_synced_folders setting doesn't experience this problem as the sync forces writable files and directories on Linux - probably because the different permission model on Linux and Windows. It took me a while to understand what was going on when I disabled syncing (to get better performance) and Drupal suddenly lost all styling ...