Ensure flexibility of environment deployment path in the generated init file

[stefanoborini]

The problem is the following: when developing multiple apps relying on the same environment, the Rprofile script does

source('.env/default/init.R')

the init file in turn does

.libPaths(c('.envs/default/lib'))

The problem is that of course if the directories are not properly positioned, this mechanism will fail.

Unfortunately R does not provide an easy way to lookup the current script path, which means that we'll have to rely on variables in the Rprofile to make this more relocatable