Update the crontab generation to produce a better file for virtualenvs.
Review Request #13118 — Created June 20, 2023 and submitted
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.