Fix write permission to renv sandbox directory to allow deletion of the project before deployment

The current deployment code first removes the existing directory structure (see the .cleanup-repo block in the .gitlab-ci.yml file). However, after upgrading the R renv package in ShaDoc, we ran into problems when deploying the code (shiny/shadoc#133). This turns out to be caused by a change in version 0.17.0 of the renv package (see here, remark about their) issue 1090) where they make the sandbox directory non-writeable. As a result, our rm -rf command for the project directory fails.