* [CI] Improve JVM test in GitHub Actions
* Use env var for Wagon options [skip ci]
* Move the retry flag to pom.xml [skip ci]
* Export env var RABIT_MOCK to run Spark tests [skip ci]
* Correct location of env var
* Re-try up to 5 times [skip ci]
* Don't run distributed training test on Windows
* Fix typo
* Update main.yml
* [CI] Move lint to GitHub Actions
* [CI] Move Doxygen to GitHub Actions
* [CI] Move Sphinx build test to GitHub Actions
* [CI] Reduce workload for Windows R tests
* [CI] Move clang-tidy to Build stage
* [CI] Move lint to a separate script
* [CI] Improved lintr launcher
* Add lintr as a separate action
* Add custom parsing logic to print out logs
* Fix lintr issues in demos
* Run R demos
* Fix CRAN checks
* Install XGBoost into R env before running lintr
* Install devtools (needed to run demos)
