Switch to r-lib/actions v2

[riccardoporreca]

This is by now the actively-maintained and improved version of r-related actions

See https://github.com/r-lib/actions/releases/tag/v2

As part of this, we can:

• Remove the fix for symlinks in troduced in CI Fix for symlinks and timeout on macOS #24 => fixed in v2, see Possible breaking changes to macos-latest runner for older versions of R r-lib/actions#412 (comment)
  • We should however keep the timeout for CI Fix for symlinks and timeout on macOS #24, since setup-r / setup-pandoc stuck on macOS (hitting 6h timeout) r-lib/actions#435 isn't fully addressed
• Drop the explicit exclusion of 32-bit installs and checks on Windows introduced in No multiarch R CMD check for r-devel on Windows #22 No multiarch package installs for r-devel on Windows #23 => This should not be an issue anymore on r-devel, see Install only 64-bit on R-devel Windows r-lib/actions#339