pak fails to install on GHA with the system python on the GHA ubuntu runner.

There may be something wonky with how R_LIBS_SITE env var setup or parsed, because dir_create() appears to be receiving a colon separated set of paths, rather than a single path. Or this may be a different manifestation of #641.

Subsequently this error message appears:

  Error in library(pak, lib.loc = .Library.site) : 
    there is no package called ‘pak’
  Execution halted

example failed run: https://github.com/t-kalinowski/guildai-r/actions/runs/3336914090/jobs/5522623455