For virtualenv-based installs, we now just hard-code the path to

rb-site, which will ensure the right Python environment is used.
For a global install, we use the logic we've had before. We could change

this to also point to the rb-site path, but the plan for now is to avoid

altering behavior that's known to work globally.

Tested the crontabs for both on two fresh sites, making sure they both

ran as expected.