Disable renv sandboxing

[mlell]

The renv R package has a feature called "sandbox" where it uses a folder with symlinks to the core R packages as system library to avoid loading other system-wide packages. This is unnecessary for this project where the system packages are frozen in the container anyway. Moreover, it prevents the BH replacement package from being recognized without further configuration.

The environment variable RENV_CONFIG_SANDBOX_ENABLED should be set to FALSE