Add cache steps to lintr workflow examples

[Moohan]

Describe the solution you'd like
The lintr workflows lint-changed-files.yaml, lint-project.yaml and lint.yaml should have steps added so that they use GitHub's cache to make use of the lintr caching.

Additional context
Something similar is done in style.yaml but I wouldn't know how to replicate that, as I don't understand enough about GitHub actions, GitHub cache or caching in general!