pkgcache 2.2.0

• New pkg.current_platform option and PKG_CURRENT_PLATFORM environment
  variable to override the detected platform.

• In repo_add() and repo_resolve() the MRAN@ prefix is now deprecated
  and resolves to PPM, because MRAN will be retired soon. See more at
  https://posit.co/blog/migrating-from-mran-to-posit-package-manager/.

• The metadata cache now has SystemRequirements information for Bioconductor
  packages.