ci: switch to GitHub Actions

The goal is for the new setup to largely follow the Drone setup, but
there are some notable differences:

 * The Drone pipelines used custom images built to track Metworx's
   Ubuntu version (currently 18.04, soon to be 20.04).  The GitHub
   Actions use GitHub-hosted runners and test with the two most recent
   Ubuntu releases available (20.04 and 22.04).

   r-lib/actions is used to install R and dependencies at the time of
   the run, with the built-in caching of setup-r-dependencies.

 * There is a build for the latest R version in addition to the R
   versions that were tested by Drone (3.6, 4.0, and 4.1).

 * For a PR, the Drone setup had two builds, one triggered by the
   branch push (testing on branch tip) and another by the PR (testing
   on merge with base).

   For the new setup, there is just one build triggered by the PR.
   It's done on the merge of the PR branch and its base.

   This means that the tip of the PR branch isn't tested.  In general,
   that's fine because the merge is what ultimately will land.  And
   for the cases where testing on the tip of the branch is useful
   (e.g., debugging a discrepancy due to a semantic merge conflict), a
   'scratch/*' branch pointing to the same spot can be pushed.

.Rbuildignore

@@ -17,6 +17,5 @@ Makefile
 ^\.Rproj\.user$
 dev
 ^docs$
-\.drone\.yml
-^\.drone\.jsonnet$
+^\.github$
 vignettes/*\.pdf