[Feature Request]: Ability to disable CRAN repo injection

[dgkf-roche]

Really just migrating from the conversation in https://github.com/r-lib/remotes/issues/790, but wanted to make sure that request didn't get lost on another repo.

It was proposed to add a global option to disable the automatic injection of a CRAN repository, similar to what is already available for BioC via options(pkg.use_bioconductor = FALSE)