[ci] [R-package] use macOS 11.x in R 4.x CI jobs

[jameslamb]

Summary

Created from #4989 (comment).

LightGBM's CI jobs for the R package should use the newest available macOS version (i.e. macOS-latest environment from GitHub Actions) together with jobs testing the newest available R version.

Motivation

macOS 11 ("Big Sur") was first introduced in June 2020 (about 19 months ago). It's likely that a large portion of the LightGBM users working on Macs are using R 4.1.x along with macOS 11. Testing this combination would prevent changes that break compatibility with such environments from being merged.

Description

To address this issue, update os: in https://github.com/microsoft/LightGBM/blob/master/.github/workflows/r_package.yml to macOS-latest for R 4.x macOS jobs.

Note that when I tried this in #4989, I saw the following error (build link):

installer: Error - the package path specified was invalid: '/Users/runner/work/LightGBM/LightGBM/R.pkg'.

Possible solutions to that:

1. maybe R 4.2.0 will produce a .pkg (at https://cran.r-project.org/bin/macosx/base/) that works with Big Sur
2. installing R from Homebrew (https://formulae.brew.sh/formula/r)
3. maybe the macOS-latest environment will become compatible with R 4.1.2 in response to Update/Add R actions/runner-images#4960

References

• Update to macos-latest to run on macOS 11 actions/runner-images#4048
• Update/Add R actions/runner-images#4960
• [ci] [R-package] macOS builds failing to install R #4988
• Possible breaking changes to macos-latest runner for older versions of R r-lib/actions#412