bioc_install_repos() deduces the URLs of the Bioconductor repositories.
Both functions observe the R_BIOC_VERSION environment variable, which
can be set to force a Bioconductor version. If this is set, then the
r_ver and bioc_ver arguments are ignored.
bioc_install_repos() observes the R_BIOC_MIRROR environment variable
and also the BioC_mirror option, which can be set to the desired
Bioconductor mirror. The option takes precedence if both are set. Its
default value is https://bioconductor.org.