The new rbinstall.process module includes two utilities, debug() and

run().
debug() will output debug information to the console when the

RBINSTALL_DEBUG environment variable is set to 1, optionally showing a

custom prefix.
run() will run a command, streaming output and raising an exception on

errors. If RBINSTALL_DRY_RUN is set to 1, then the command won't

actually be run.

Successfully tested these with the rest of the installer.