Disable sandbox in renv

[MichalLauer]

When I use {renv}, I often work with a lot of packages and I get the following warning in Github Action logs:

Warning message:
renv took longer than expected (20 seconds) to activate the sandbox.

The sandbox can be disabled by setting:

    RENV_CONFIG_SANDBOX_ENABLED = FALSE

within an appropriate start-up .Renviron file.

This message is not much helpful, because the .Renviron file can't be uploaded to Github as it may contain sensitive data. This is why I propose a new parameter - sandbox - which sets the environmental variable after {renv} is activated.

I hope I am merging this into the correct branch - if anything, feel free to let me know :)