Ci/speedup dependency install

[timcadman]

Background

CI takes a long time to run which makes it a bit frustrating when trying to debug problems

What's changed

Dependencies

• Install R packages as precompiled binaries from Posit Public Package Manager instead of compiling from source. Reduces dependency install step to ~4 mins.
• Add a Cache@2 step for the R site-library, and only install packages not already present, so cached deps are reused across runs.

Testthat

• Added option to just log in and out once per test run rather than per file.