GHA CI cleanups: Use action for installing rtools, restrict parallelism

[andrjohns]

Summary

This PR adds two changes to the current GHA checks workflow:

• Use r-lib/actions/setup-r to handle the RTools installation, as it now supports aarch64 RTools
• Restrict the number of parallel tasks per job to 5

The current CI saturates the maximum number of concurrent jobs for the stan-dev org, blocking CI in other repos.

Tests

N/A - current tests should still pass

Side Effects

N/A

Release notes

Updated installation method for RTools in CI, restricted the maximum parallelism of CI runs

Checklist

• Copyright holder: Andrew Johnson

  The copyright holder is typically you or your assignee, such as a university or company. By submitting this pull request, the copyright holder is agreeing to the license the submitted work under the following licenses:
  - Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
  - Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

• the basic tests are passing

  • unit tests pass (to run, use: ./runTests.py test/unit)
  • header checks pass, (make test-headers)
  • dependencies checks pass, (make test-math-dependencies)
  • docs build, (make doxygen)
  • code passes the built in C++ standards checks (make cpplint)

• the code is written in idiomatic C++ and changes are documented in the doxygen

• the new changes are tested