Add libuv1-dev system dependency required by fs 2.x

[donny-wong]

The fs R package 2.x dropped its vendored copy of libuv and now requires
a system libuv development library, discovered via pkg-config at build time.
Without libuv1-dev installed, creating an R tester environment fails.

Add libuv1-dev to the apt-get install block in the R tester setup script
alongside the other system dependencies.

Test Setup

How fs is loaded

testthat → pkgload → fs

The instructor test script is responsible for loading testthat.

# instructor test script
library(testthat)

Verifying the setup

After saving the autotest settings, run a test. If none of the following errors appear, the setup is working correctly:

Package libuv was not found in the pkg-config search path.
Package 'libuv', required by 'virtual:world', not found
ERROR: configuration failed for package 'fs'