On every PR check tiltPlot on CI

Relates to

https://github.com/2DegreesInvesting/tiltPlot/pull/79

This PR adds a GitHub-Actions workflow to run R CMD check on tiltPlot on each PR to tiltIndicator. This alerts potential breaking changes in tiltPlot from upstream dependencies.

This follows Google's Beyoncé Rule

If you liked it, then you shoulda put a test on it.”

TODO

[x] Link related issue/PR.
[x] Describe the goal of the PR. Avoid details that are clear in the diff.
[x] Mark the PR as draft.
[x] Include a unit test.
[x] Review your own PR in "Files changed".
[x] Ensure the PR branch is updated.
[x] Ensure the checks pass.
[x] Change the status from draft to ready.
[x] Polish the PR title and description.
[ ] Assign a reviewer.

EXCEPTIONS

[ ] Slide here any item that you intentionally choose to not do.

2DegreesInvesting / tiltIndicator

On every PR check tiltPlot on CI #721